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When the affinity union grows, > the per-CPU threshold is recalculated and the MM can switch back to > per-task ownership through deferred work. > > mm_cid_calc_pcpu_thrs() incorrectly uses > min(nr_cpus_allowed, users) to calculate that switch-back threshold. > This makes pcpu_thrs no larger than users, so the > users < pcpu_thrs check in mm_update_cpus_allowed() can never succeed. > > For example, with three users restricted to two CPUs, the MM enters > per-CPU mode. On an eight-CPU system, widening the affinity union to > all eight CPUs should produce a threshold of four, but the old > calculation produces three and returns early. The MM therefore remains > in per-CPU mode even though the widened CPU set makes per-task > ownership preferable. > > Calculate pcpu_thrs from nr_cpus_allowed directly. The users bound in > __mm_update_max_cids() still controls when per-CPU mode is entered, so > this does not change the threshold calculated when switching from > per-task to per-CPU ownership. > > Tested with an x86_64 QEMU reproducer using three users, an initial > two-CPU affinity, and an eight-CPU affinity expansion. The baseline > did not invoke mm_cid_work_fn() after the affinity expansion. The > fixed kernel invoked the deferred work once and completed the switch > back to per-task mode. I think you're onto something. We should cover those scenarios with selftests in tools/testing/selftests/rseq/. Thomas, I recall you had tests for those state transitions when rewriting rseq mm_cid. Were those local or did they make it upstream ? Thanks, Mathieu > > Fixes: fbd0e71dc370 ("sched/mmcid: Provide CID ownership mode fixup functions") > Signed-off-by: Hui Su > --- > kernel/sched/core.c | 5 ++--- > 1 file changed, 2 insertions(+), 3 deletions(-) > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > index 7885ff76e69f..0280ed132d0a 100644 > --- a/kernel/sched/core.c > +++ b/kernel/sched/core.c > @@ -10845,11 +10845,10 @@ static void __mm_update_max_cids(struct mm_mm_cid *mc) > > static inline unsigned int mm_cid_calc_pcpu_thrs(struct mm_mm_cid *mc) > { > - unsigned int opt_cids; > + unsigned int nr_cpus = mc->nr_cpus_allowed; > > - opt_cids = min(mc->nr_cpus_allowed, mc->users); > /* Has to be at least 1 because 0 indicates PCPU mode off */ > - return max(min(opt_cids - opt_cids / 4, num_possible_cpus() / 2), 1); > + return max(min(nr_cpus - nr_cpus / 4, num_possible_cpus() / 2), 1); > } > > static bool mm_update_max_cids(struct mm_struct *mm) > > base-commit: 587858367581b9c55c3690f4e63382ad622719d4 -- Mathieu Desnoyers EfficiOS Inc. https://www.efficios.com