Date: Fri, 26 Jun 2026 01:45:56 +0000 From: Olivier Certner <olce@FreeBSD.org> To: src-committers@FreeBSD.org, dev-commits-src-all@FreeBSD.org, dev-commits-src-main@FreeBSD.org Subject: git: 961f48142868 - main - sched_ule: Fix off by one in preempt_thresh definition Message-ID: <6a3dd9d4.426cb.6203f418@gitrepo.freebsd.org>
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The branch main has been updated by olce: URL: https://cgit.FreeBSD.org/src/commit/?id=961f4814286820f242d8d5407b9fd7238e896936 commit 961f4814286820f242d8d5407b9fd7238e896936 Author: Olivier Certner <olce@FreeBSD.org> AuthorDate: 2026-06-19 01:51:32 +0000 Commit: Olivier Certner <olce@FreeBSD.org> CommitDate: 2026-06-26 01:43:46 +0000 sched_ule: Fix off by one in preempt_thresh definition Since 'preempt_thresh' is set to PRI_MIN_KERN by default, and comparison of the considered thread's priority with that threshold is done with '<=', PRI_MIN_KERN threads actually can preempt other threads, contrary to other non-interrupt kernel ones (between PRI_MIN_KERN + 1 and PRI_MAX_KERN). So, replace the comparison operator '<=' by '<'. The alternative would be to change the default value, but changing the comparison instead has the benefit to be consistent with the 0 setting (which forbids preemption entirely), since allowing only threads with priority 0 to preempt becomes possible. Consequently, we also change the default value for the FULL_PREEMPTION option by adding 1 to PRI_MAX_IDLE (in practice, that does not make any difference in the current setting, since no preemption will happen if the new priority value is not strictly lower than the current one, and PRI_MAX_IDLE is PRI_MAX, the highest possible priority). Reviewed by: markj Fixes: ae7a6b38d53f ("ULE 3.0: Fine grain scheduler locking and affinity improvements. (...)") MFC after: 2 weeks Event: Halifax Hackathon 202606 Location: jrm@'s dining room Sponsored by: The FreeBSD Foundation Differential Revision: https://reviews.freebsd.org/D57828 --- sys/kern/sched_ule.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/sys/kern/sched_ule.c b/sys/kern/sched_ule.c index c8d2712b89ef..eb00072b5012 100644 --- a/sys/kern/sched_ule.c +++ b/sys/kern/sched_ule.c @@ -236,7 +236,7 @@ static int __read_mostly sched_slice = 10; /* reset during boot. */ static int __read_mostly sched_slice_min = 1; /* reset during boot. */ #ifdef PREEMPTION #ifdef FULL_PREEMPTION -static int __read_mostly preempt_thresh = PRI_MAX_IDLE; +static int __read_mostly preempt_thresh = PRI_MAX_IDLE + 1; #else static int __read_mostly preempt_thresh = PRI_MIN_KERN; #endif @@ -473,7 +473,7 @@ sched_shouldpreempt(int pri, int cpri, int remote) /* * Preempt if we exceed the threshold. */ - if (pri <= preempt_thresh) + if (pri < preempt_thresh) return (1); /* * If we're interactive or better and there is non-interactivehome | help
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