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0e4c830bc1
In our codebase, `cond_resched() and `schedule()` are Linux kernel
functions that have replaced the OpenSolaris `kpreempt()` functions in
the codebase to such an extent that `kpreempt()` in zfs_context.h was
broken. Nobody noticed because we did not actually use it. The header
had defined `kpreempt()` as `yield()`, which works on OpenSolaris and
Illumos where `sched_yield()` is a wrapper for `yield()`, but that does
not work on any other platform.
The FreeBSD platform specific code implemented shims for these, but the
shim for `schedule()` forced us to wait, which is different than merely
rescheduling to another thread as the original Linux code does, while
the shim for `cond_resched()` had the same definition as its kernel
kpreempt() shim.
After studying this, I have concluded that we should reintroduce the
kpreempt() function in platform independent code with the following
definitions:
- In the Linux kernel:
kpreempt(unused) -> cond_resched()
- In the FreeBSD kernel:
kpreempt(unused) -> kern_yield(PRI_USER)
- In userspace:
kpreempt(unused) -> sched_yield()
In userspace, nothing changes from this cleanup. In the kernels, the
function `fm_fini()` will now call `kern_yield(PRI_USER)` on FreeBSD and
`cond_resched()` on Linux. This is instead of `pause("schedule", 1)` on
FreeBSD and `schedule()` on Linux. This makes our behavior consistent
across platforms.
Note that Linux's SPL continues to use `cond_resched()` and
`schedule()`. However, those functions have been removed from both the
FreeBSD code and userspace code.
This should have the benefit of making it slightly easier to port the
code to new platforms by making how things should be mapped less
confusing.
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Neal Gompa <ngompa@datto.com>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #13845
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