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fix a thread preemption regression in schedulers introduced in r270423

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Commit r270423 fixed a regression in sched_yield() that was introduced
in earlier changes.  Unfortunately, at the same time it introduced an
new regression.  The problem is that SWT_RELINQUISH (6), like all other
SWT_* constants and unlike SW_* flags, is not a bit flag.  So, (flags &
SWT_RELINQUISH) is true in cases where that was not really indended,
for example, with SWT_OWEPREEMPT (2) and SWT_REMOTEPREEMPT (11).

A straight forward fix would be to use (flags & SW_TYPE_MASK) ==
SWT_RELINQUISH, but my impression is that the switch types are designed
mostly for gathering statistics, not for influencing scheduling
decisions.

So, I decided that it would be better to check for SW_PREEMPT flag
instead.  That's also the same flag that was checked before r239157.
I double-checked how that flag is used and I am confident that the flag
is set only in the places where we really have the preemption:
- critical_exit + td_owepreempt
- sched_preempt in the ULE scheduler
- sched_preempt in the 4BSD scheduler

Reviewed by:	kib, mav
MFC after:	4 days
Sponsored by:	Panzura
Differential Revision: https://reviews.freebsd.org/D9230
This commit is contained in:
Andriy Gapon 2017-01-19 18:46:41 +00:00
parent 1c07d69bc2
commit ad9dadc437
Notes: svn2git 2020-12-20 02:59:44 +00:00 
svn path=/head/; revision=312426
2 changed files with 4 additions and 4 deletions
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4
sys/kern/sched_4bsd.c
View File

@ -968,8 +968,8 @@ sched_switch(struct thread *td, struct thread *newtd, int flags)
sched_load_rem();
td->td_lastcpu = td->td_oncpu;
preempted = !((td->td_flags & TDF_SLICEEND) ||
(flags & SWT_RELINQUISH));
preempted = (td->td_flags & TDF_SLICEEND) == 0 &&
(flags & SW_PREEMPT) != 0;
td->td_flags &= ~(TDF_NEEDRESCHED | TDF_SLICEEND);
td->td_owepreempt = 0;
td->td_oncpu = NOCPU;

4
sys/kern/sched_ule.c
View File

@ -1898,8 +1898,8 @@ sched_switch(struct thread *td, struct thread *newtd, int flags)
ts->ts_rltick = ticks;
td->td_lastcpu = td->td_oncpu;
td->td_oncpu = NOCPU;
preempted = !((td->td_flags & TDF_SLICEEND) ||
(flags & SWT_RELINQUISH));
preempted = (td->td_flags & TDF_SLICEEND) == 0 &&
(flags & SW_PREEMPT) != 0;
td->td_flags &= ~(TDF_NEEDRESCHED | TDF_SLICEEND);
td->td_owepreempt = 0;
if (!TD_IS_IDLETHREAD(td))