Reviewed-by: Tim Chase <tim@chase2k.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Elling <Richard.Elling@RichardElling.com>
Closes#652Closes#651
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Signed-off-by: gaurkuma <gauravk.18@gmail.com>
Closes#641
This was probably accidentally committed in
aeb9baa618
Fix: handle NULL case in spl_kmem_free_track()
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Gvozden Neskovic <neskovic@gmail.com>
Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>
Closes#644
taskq work item to more than one queue concurrently. Also, please
see discussion in zfsonlinux/zfs#3840.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Boris Protopopov <boris.protopopov@actifio.com>
Closes#609
taskq_seq_show_impl walks the tq_active_list to show the tqent_func and
tqent_arg. However for taskq_dispatch_ent, it's very likely that the
task entry will be freed during the function call, and causes a
use-after-free bug.
To fix this, we duplicate the task entry to an on-stack struct, and
assign it instead to tqt_task. This way, the tq_lock alone will
guarantee its safety.
Reviewed-by: Tim Chase <tim@chase2k.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Chunwei Chen <david.chen@osnexus.com>
Closes#638Closes#640
gcc-7 seems to use __udivmoddi4 for 64-bit division on 32-bit arch. This
patch implement them so we don't get undefined reference error.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: loli10K <ezomori.nozomu@gmail.com>
Signed-off-by: Chunwei Chen <david.chen@osnexus.com>
Closes zfsonlinux/zfs#6417
Closes#636
In unattended operations it's often more useful to have node
panic and reboot when it encounters problems as opposed to
sit there indefinitely waiting for somebody to discover it.
This implements an spl_panic_crash module parameter, set it
to nonzero to cause spl_panic() to call panic().
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Closes#634
When we load a ZFS pool having spa_name equals to some existing kstat
we would have to create a duplicate entry, which procfs doesn't like.
For instance a ZFS pool named "zil" would have its kstat "txgs"
(module "zfs/zil") intalled under "/proc/spl/kstat/zfs/zil":
unfortunately we already have a kstat named "zil" (module "zfs")
installed in the same procfs location.
Avoid this issue by skipping the duplicate entry creation in procfs.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes#628
Historically the SPL cached the system hostid the first time it
was accessed. This was done to speed up subsequent accesses.
But in practice the system host id is rarely accessed and its
inconvenient that it doesn't promptly detect /etc/hostid
configuration changes. Therefore, zone_get_hostid() has been
updated to always refresh the system hostid reported.
Reviewed-by: Olaf Faaland <faaland1@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes#626
Exclude Makefile.in in module/ and fix the gitignore in cmd/
Also, ignore *.patch and *.orig files
Signed-off-by: Chunwei Chen <david.chen@osnexus.com>
Perform the already past expiration time check before updating
cvp->cv_mutex with the provided mutex. This check only depends
on local state. Doing it first ensures that cvp->cv_mutex will not
be updated in the timeout case or if it's ever called with an
expire_time <= now.
Reviewed-by: Tim Chase <tim@chase2k.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes#616
Change SPL_FSTRANS to optionally contains PF_FSTRANS. Also, add
__spl_pf_fstrans_check for the checks specifically for PF_FSTRANS.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Chunwei Chen <david.chen@osnexus.com>
Closes#614
Before kernel 2.6.29 credentials were embedded in task_structs, and zfs had
cases where one thread would need to refer to the credential of another thread,
forcing it to take a hold on the foreign thread's task_struct to ensure it was
not freed.
Since 2.6.29, the credential has been moved out of the task_struct into a
cred_t.
In addition, the mainline kernel originally did not export __put_task_struct()
but the RHEL5 kernel did, according to zfsonlinux/spl@e811949a57. As of
2.6.39 the mainline kernel exports it.
There is no longer zfs code that takes or releases holds on a task_struct, and
so there is no longer any reference to __put_task_struct().
This affects the linux 4.11 kernel because the prototype for
__put_task_struct() is in a new include file (linux/sched/task.h) and so the
config check failed to detect the exported symbol.
Removing the unnecessary stub and corresponding config check. This works on
kernels since the oldest one currently supported, 2.6.32 as shipped with
Centos/RHEL.
Reviewed-by: Chunwei Chen <david.chen@osnexus.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Olaf Faaland <faaland1@llnl.gov>
Closes#608
There are changes to vfs_getattr() in torvalds/linux@a528d35. The new
interface is:
int vfs_getattr(const struct path *path, struct kstat *stat,
u32 request_mask, unsigned int query_flags)
The request_mask argument indicates which field(s) the caller intends to
use. Fields the caller does not specify via request_mask may be set in
the returned struct anyway, but their values may be approximate.
The query_flags argument indicates whether the filesystem must update
the attributes from the backing store.
This patch uses the query_flags which result in vfs_getattr behaving the same
as it did with the 2-argument version which the kernel provided before
Linux 4.11.
Members blksize and blocks are now always the same size regardless of
arch. They match the size of the equivalent members in vnode_t.
The configure checks are modified to ensure that the appropriate
vfs_getattr() interface is used.
A more complete fix, removing the ZFS dependency on vfs_getattr()
entirely, is deferred as it is a much larger project.
Reviewed-by: Chunwei Chen <david.chen@osnexus.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Olaf Faaland <faaland1@llnl.gov>
Closes#608
Replace uses of set_task_state(current, STATE) with
set_current_state(STATE).
In Linux 4.11, torvalds/linux@642fa44, set_task_state() is removed.
All spl uses are of the form set_task_state(current, STATE).
set_current_state(STATE) is equivalent and has been available since
Linux 2.2.26.
Furthermore, set_current_state(STATE) is already used in about 15
locations within spl. This change should have no impact other than
removing an unnecessary dependency.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Olaf Faaland <faaland1@llnl.gov>
Closes#603
Resolve a false positive in the kmemleak checker by shifting to the
kernel slab. It shows up because vn_file_cache is using KMC_KMEM
which is directly allocated using __get_free_pages, which is not
automatically tracked by kmemleak.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Chunwei Chen <david.chen@osnexus.com>
Closes#599
SLAB_USERCOPY flag was used to indicate PAX
not to kill copies from kernel to userland.
With recent grsecurity patchset and
CONFIG_GRKERNSEC_HIDESYM that enables
CONFIG_PAX_USERCOPY zfs would panic.
Handle newer API while keeping old one functional.
Tested-by: RageLtMan <rageltman@sempervictus>
Reviewed-by: spendergrsec <spender@grsecurity.net>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Kevin Tanguy <kevin.tanguy@ovh.net>
Closes#595
When building SPL within the kernel tree, C99 initializers cause
build failures and need to be converted to C89 as kernel CFLAGS
specify -std=gnu89.
This fix was provided by @behlendorf in #595 discussion notes and
manually implemented in the current master revision.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: RageLtMan <rageltman@sempervictus>
Closes#597
The main complication from the RT patch set is that the RW semaphore
locks change such that read locks on an rwsem can be taken only by
a single thread. All other threads are locked out. This single
thread can take a read lock multiple times though. The underlying
implementation changes to a mutex with an additional read_depth
count.
The implementation can be best understood by inspecting the RT
patch. rwsem_rt.h and rt.c give the best insight into how RT
rwsem works. My implementation for rwsem_tryupgrade is basically
an inversion of rt_downgrade_write found in rt.c. Please see the
comments in the code.
Unfortunately, I have to drop SPLAT rwlock test4 completely as this
test tries to take multiple locks from different threads, which RT
rwsems do not support. Otherwise SPLAT, zconfig.sh, zpios-sanity.sh
and zfs-tests.sh pass on my Debian-testing VM with the kernel
linux-image-4.8.0-1-rt-amd64.
Tested-by: kernelOfTruth <kerneloftruth@gmail.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Clemens Fruhwirth <clemens@endorphin.org>
Closes zfsonlinux/zfs#5491
Closes#589Closes#308
Refactor the code by making splat_test_{init,fini}, splat_subsystem_{init,fini}
into functions. They don't have reason to be macro and it would be too bloated
to inline every call.
Signed-off-by: Chunwei Chen <david.chen@osnexus.com>
Add a dedicated system_delay_taskq for long delay like spa_deadman and
zpl_posix_acl_free. This will allow us to use system_taskq in the manner of
dispatch multiple tasks and call taskq_wait_outstanding.
Reviewed by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Chunwei Chen <david.chen@osnexus.com>
Closes#588
To prevent holding tq_lock for too long.
Before zfsonlinux/zfs@8e71ab9, hogging delay tasks and cat /proc/spl/taskq
would easily cause a lockup. While that bug has been fixed. It's probably
still a good idea to do this just in case task lists grow too large.
Reviewed-by: Tim Chase <tim@chase2k.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Chunwei Chen <david.chen@osnexus.com>
Closes#586
Add the TASKQID_INVALID and TASKQID_INITIAL macros and update the
taskq implementation and test cases to use them. This is solely
for the purposes of readability and introduces no functional change.
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Add a minimal implementation of vmem_size() which accounts for the
virtual memory usage of the SPL's kmem cache. This functionality
is only useful on 32-bit systems with a small virtual address space.
The following assumptions are made:
1) The major SPL consumer of virtual memory is the kmem cache.
2) Memory allocated with vmem_alloc() is short lived and can be ignored.
3) Allow a 4MB floor as a generous pad given normal consumption.
4) The spl_kmem_cache_sem only contends with cache create/destroy.
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
In Linux 4.9, torvalds/linux@81243ea, group_info changed from 2d array via
->blocks to 1d array via ->gid. We change the spl cred functions accordingly.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Chunwei Chen <david.chen@osnexus.com>
Closes#581
No need to crhold current_cred(), fix possible leak in splat_cred_test2
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Chunwei Chen <david.chen@osnexus.com>
Closes#556
init_groups has 0 nblocks, therefore calling the current crgetgroups with
init_groups would result in out-of-bound access. We fix this by returning NULL
when nblocks is 0.
Cap crgetngroups to NGROUPS_PER_BLOCK, since crgetgroups will only return
blocks[0].
Also, remove all get_group_info. The cred already holds reference on the
group_info, and cred is not mutable. So there's no reason to hold extra
reference, if we hold cred.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Chunwei Chen <david.chen@osnexus.com>
Closes#556
When iterating per_cpu values, we need to use for_each_possible_cpu. While
NR_CPUS indicates the number of CPU supported by the kernel, it might not
initialize all of them if the kernel decides it's not possible to use them.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Chunwei Chen <david.chen@osnexus.com>
Closes#578
Due to changes in the task_struct the following warning is occurs
when initializing the global p0. Since this structure only exists
for it's address to be taken initialize it in a manor which isn't
sensitive to internal changes to the structure.
module/spl/spl-generic.c:58:1: error: missing braces around
initializer [-Werror=missing-braces]
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes#576
Explicitly cast type in splat-rwlock.c test case to silence
the following warning.
warning: format ‘%ld’ expects argument of type ‘long int’,
but argument N has type ‘int’
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes#574
In order to support ABD with large blocks the spl_kmem_alloc_warn
limit needs to be increased to 64K.
A 16M block requires that pointers be stored for 4096 4K-pages
on an x86_64 system. Each of these pointers is 8 bytes requiring
an allocation of 8*4096=32,768 bytes. The addition of a small
header to this structure pushes the allocation over the default
32K warning threshold.
In addition, fix a small bug where MAX was used instead of MIN
when setting the default. This ensures a reasonable limit is
still set on systems with page sizes larger then 4K.
Reviewed-by: David Quigley <david.quigley@intel.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes#571
When DEBUG_KMEM_TRACKING is enabled in SPL, we keep tracking all
the buffers alloced by kmem_alloc() and kmem_zalloc(). If a NULL
pointer which indicates no track info in SPL is passed to
spl_kmem_free_track, we just ignore it.
Signed-off-by: GeLiXin <ge.lixin@zte.com.cn>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue zfsonlinux/zfs#4967
Closes#567
For non-rwsem-spinlocks the "count" member was changed from a
"long" to "atomic_long_t" type. A configure check has been
added to detect this change along with new versions of the
_rwsem_tryupgrade() function and RWSEM_COUNT() macro. See
https://github.com/torvalds/linux/commit/8ee62b18 for complete
details.
Signed-off-by: Tim Chase <tim@chase2k.com>
Signed-off-by: Chunwei Chen <david.chen@osnexus.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes#563
The policy is to try to allocate with KM_NOSLEEP, which will lead to
memory allocation with GFP_ATOMIC, and if it fails, it will launch
an taskq to expand slab space.
This way it should be able to get better NUMA memory locality and
reduce the overhead of context switch.
Signed-off-by: Jinshan Xiong <jinshan.xiong@gmail.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes#551
This splat_vprint is using tq_arg->name after tq_arg is freed.
Signed-off-by: Chunwei Chen <david.chen@osnexus.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes#557
Current rw_tryupgrade does rw_exit and then rw_tryenter(RW_RWITER), and then
does rw_enter(RW_READER) if it fails. This violate the assumption that
rw_tryupgrade should be atomic and could cause extra contention or even lock
inversion.
This patch we implement a proper rw_tryupgrade. For rwsem-spinlock, we take
the spinlock to check rwsem->count and rwsem->wait_list. For normal rwsem, we
use cmpxchg on rwsem->count to change the value from single reader to single
writer.
Signed-off-by: Chunwei Chen <david.chen@osnexus.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tim Chase <tim@chase2k.com>
Closes zfsonlinux/zfs#4692
Closes#554
wait_event is a macro, so the current implementation will cause re-
evaluation of tq_next_id every time it wakes up. This would cause
taskq_wait_outstanding(tq, 0) to be equivalent to taskq_wait(tq)
Signed-off-by: Chunwei Chen <david.chen@osnexus.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tim Chase <tim@chase2k.com>
Issue #553
While taskq_destroy would wait for dynamic_taskq to finish its tasks, but it
does not implies the thread being spawned is up and running. This will cause
taskq to be freed before the thread can exit.
We fix this by using tq_nspawn to indicate how many threads are being spawned
before they are inserted to the thread list. And have taskq_destroy to wait
for it to drop to zero.
Signed-off-by: Chunwei Chen <david.chen@osnexus.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tim Chase <tim@chase2k.com>
Issue #553Closes#550
In 39cd90e, I mistakenly disabled the ability of using absolute expire time in
cv_timedwait_hires. I don't quite sure why I did that, so let's restore it.
Signed-off-by: Chunwei Chen <david.chen@osnexus.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tim Chase <tim@chase2k.com>
Issue #553
Linux 4.7 changes i_mutex to i_rwsem, and we should used inode_lock and
inode_lock_shared to do exclusive and shared lock respectively.
We use spl_inode_lock{,_shared}() to hide the difference. Note that on older
kernel you'll always take an exclusive lock.
We also add all other inode_lock friends. And nested users now should
explicitly call spl_inode_lock_nested with correct subclass.
Signed-off-by: Chunwei Chen <david.chen@osnexus.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue zfsonlinux/zfs#4665
Closes#549
The problem described in 2a5d574 also applies to XFS's file or inode
fallocate method. Both paths may trigger writeback and expose this
issue, see the full stack below.
When layered on XFS a warning will be emitted under CentOS7 when entering
either the file or inode fallocate method with PF_FSTRANS already set.
To avoid triggering this error PF_FSTRANS is cleared and then reset
in vn_space().
WARNING: at fs/xfs/xfs_aops.c:982 xfs_vm_writepage+0x58b/0x5d0
Call Trace:
[<ffffffff810a1ed5>] warn_slowpath_common+0x95/0xe0
[<ffffffff810a1f3a>] warn_slowpath_null+0x1a/0x20
[<ffffffffa0231fdb>] xfs_vm_writepage+0x58b/0x5d0 [xfs]
[<ffffffff81173ed7>] __writepage+0x17/0x40
[<ffffffff81176f81>] write_cache_pages+0x251/0x530
[<ffffffff811772b1>] generic_writepages+0x51/0x80
[<ffffffffa0230cb0>] xfs_vm_writepages+0x60/0x80 [xfs]
[<ffffffff81177300>] do_writepages+0x20/0x30
[<ffffffff8116a5f5>] __filemap_fdatawrite_range+0xb5/0x100
[<ffffffff8116a6cb>] filemap_write_and_wait_range+0x8b/0xd0
[<ffffffffa0235bb4>] xfs_free_file_space+0xf4/0x520 [xfs]
[<ffffffffa023cbce>] xfs_file_fallocate+0x19e/0x2c0 [xfs]
[<ffffffffa036c6fc>] vn_space+0x3c/0x40 [spl]
[<ffffffffa0434817>] vdev_file_io_start+0x207/0x260 [zfs]
[<ffffffffa047170d>] zio_vdev_io_start+0xad/0x2d0 [zfs]
[<ffffffffa0474942>] zio_execute+0x82/0xe0 [zfs]
[<ffffffffa036ba7d>] taskq_thread+0x28d/0x5a0 [spl]
[<ffffffff810c1777>] kthread+0xd7/0xf0
[<ffffffff8167de2f>] ret_from_fork+0x3f/0x70
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tim Chase <tim@chase2k.com>
Signed-off-by: Nikolay Borisov <kernel@kyup.com>
Closeszfsonlinux/zfs#4529
When a TQ_NOQUEUE dispatch is done on a dynamic taskq, allow another
thread to be spawned. This will cause TQ_NOQUEUE to behave similarly
as it does with non-dynamic taskqs.
Add support for TQ_NOQUEUE to taskq_dispatch_ent().
Signed-off-by: Tim Chase <tim@onlight.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes#530
This implementation of rw_tryupgrade() behaves slightly differently
from its counterparts on other platforms. It drops the RW_READER lock
and then acquires the RW_WRITER lock leaving a small window where no
lock is held. On other platforms the lock is never released during
the upgrade process. This is necessary under Linux because the kernel
does not provide an upgrade function.
There are currently no callers in the ZFS code where this change in
behavior is a problem. In fact, in most cases the code is already
written such that if the upgrade fails the RW_READER lock is dropped
and the caller blocks waiting to acquire the lock as RW_WRITER.
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tim Chase <tim@chase2k.com>
Signed-off-by: Matthew Thode <prometheanfire@gentoo.org>
Closes zfsonlinux/zfs#4388
Closes#534
Perf profiling of dd on a zvol revealed that my system spent 3.16% of
its time in random_get_pseudo_bytes(). No SPL consumers need
cryptographic strength entropy, so we can reduce our overhead by
changing the implementation to utilize a fast PRNG.
The Linux kernel did not export a suitable PRNG function until it
exported get_random_int() in Linux 3.10. While we could implement an
autotools check so that we use it when it is available or even try to
access the symbol on older kernels where it is not exported using the
fact that it is exported on newer ones as justification, we can instead
implement our own pseudo-random data generator. For this purpose, I have
written one based on a 128-bit pseudo-random number generator proposed
in a paper by Sebastiano Vigna that itself was based on work by the late
George Marsaglia.
http://vigna.di.unimi.it/ftp/papers/xorshiftplus.pdf
Profiling the same benchmark with an earlier variant of this patch that
used a slightly different generator (roughly same number of
instructions) by the same author showed that time spent in
random_get_pseudo_bytes() dropped to 0.06%. That is a factor of 50
improvement. This particular generator algorithm is also well known to
be fast:
http://xorshift.di.unimi.it/#speed
The benchmark numbers there state that it runs at 1.12ns/64-bits or 7.14
GBps of throughput on an Intel Core i7-4770 in what is presumably a
single-threaded context. Using it in `random_get_pseudo_bytes()` in the
manner I have will probably not reach that level of performance, but it
should be fairly high and many times higher than the Linux
`get_random_bytes()` function that we use now, which runs at 16.3 MB/s
on my Intel Xeon E3-1276v3 processor when measured by using dd on
/dev/urandom.
Also, putting this generator's seed into per-CPU variables allows us to
eliminate overhead from both spin locks and CPU memory barriers, which
is NUMA friendly.
We could have alternatively modified consumers to use something like
`gethrtime() % 3` as suggested by both Matthew Ahrens and Tim Chase, but
that has a few potential problems that this approach avoids:
1. Switching to `gethrtime() % 3` in hot code paths today requires
diverging from illumos-gate and does nothing about potential future
patches from illumos-gate that call our slow `random_get_pseudo_bytes()`
in different hot code paths. Reimplementing `random_get_pseudo_bytes()`
with a per-CPU PRNG avoids both of those things entirely, which means
less work for us in the future.
2. Looking at the code that implements `gethrtime()`, I think it is
unlikely to be faster than this per-CPU PRNG implementation of
`random_get_pseudo_bytes()`. It would be best to go with something fast
now so that there is no point in revisiting this from a performance
perspective.
3. `gethrtime() % 3` can vary in behavior from system to system based on
kernel version, architecture and clock source. In comparison, this
per-CPU PRNG is about ~40 lines of code in `random_get_pseudo_bytes()`
that should behave consistently across all systems regardless of kernel
version, system architecture or machine clock source. It is unlikely
that we would ever need to revisit this per-CPU PRNG while the same
cannot be said for `gethrtime() % 3`.
4. `gethrtime()` uses CPU memory barriers and maybe atomic instructions
depending on the clock source, so replacing `random_get_pseudo_bytes()`
with `gethrtime()` in hot code paths could still require a future person
working on NUMA scalability to reimplement it anyway while this per-CPU
PRNG would not by virtue of using neither CPU memory barriers nor atomic
instructions. Note that I did not check various clock sources for the
presence of atomic instructions. There is simply too much code to read
and given the drawbacks versus this per-cpu PRNG, there is no point in
being certain.
5. I have heard of instances where poor quality pseudo-random numbers
caused problems for HPC code in ways that took more than a year to
identify and were remedied by switching to a higher quality source of
pseudo-random numbers. While filesystems are different than HPC code, I
do not think it is impossible for us to have instances where poor
quality pseudo-random numbers can cause problems. Opting for a well
studied PRNG algorithm that passes tests for statistical randomness over
changing callers to use `gethrtime() % 3` bypasses the need to think
about both whether poor quality pseudo-random numbers can cause problems
and the statistical quality of numbers from `gethrtime() % 3`.
6. `gethrtime()` calls `getrawmonotonic()`, which uses seqlocks. This is
probably not a huge issue, but anyone using kgdb would never be able to
step through a seqlock critical section, which is not a problem either
now or with the per-CPU PRNG:
https://en.wikipedia.org/wiki/Seqlock
The only downside that I can see is that this code's memory requirement
is O(N) where N is NR_CPUS, versus the current code and `gethrtime() %
3`, which are O(1), but that should not be a problem. The seeds will use
64KB of memory at the high end (i.e `NR_CPU == 4096`) and 16 bytes of
memory at the low end (i.e. `NR_CPU == 1`). In either case, we should
only use a few hundred bytes of code for text, especially since
`spl_rand_jump()` should be inlined into `spl_random_init()`, which
should be removed during early boot as part of "Freeing unused kernel
memory". In either case, the memory requirements are minuscule.
Signed-off-by: Richard Yao <ryao@gentoo.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tim Chase <tim@chase2k.com>
Closes#372
