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Message-ID: <43da8f9d-f0fd-d67b-7384-fc03ad159f29@redhat.com> Date: Fri, 27 Jan 2023 09:54:26 -0500 From: Waiman Long <longman@...hat.com> To: Will Deacon <will@...nel.org> Cc: Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>, Juri Lelli <juri.lelli@...hat.com>, Vincent Guittot <vincent.guittot@...aro.org>, Dietmar Eggemann <dietmar.eggemann@....com>, Steven Rostedt <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>, Daniel Bristot de Oliveira <bristot@...hat.com>, Valentin Schneider <vschneid@...hat.com>, Phil Auld <pauld@...hat.com>, Linus Torvalds <torvalds@...ux-foundation.org>, linux-kernel@...r.kernel.org Subject: Re: [PATCH v3] sched: Store restrict_cpus_allowed_ptr() call state On 1/27/23 07:59, Will Deacon wrote: > Hi Waiman, > > On Thu, Jan 26, 2023 at 08:55:27PM -0500, Waiman Long wrote: >> The user_cpus_ptr field was originally added by commit b90ca8badbd1 >> ("sched: Introduce task_struct::user_cpus_ptr to track requested >> affinity"). It was used only by arm64 arch due to possible asymmetric >> CPU setup. >> >> Since commit 8f9ea86fdf99 ("sched: Always preserve the user requested >> cpumask"), task_struct::user_cpus_ptr is repurposed to store user >> requested cpu affinity specified in the sched_setaffinity(). >> >> This results in a slight performance regression on an arm64 >> system when booted with "allow_mismatched_32bit_el0" >> on the command-line. The arch code will (amongst >> other things) calls force_compatible_cpus_allowed_ptr() and >> relax_compatible_cpus_allowed_ptr() when exec()'ing a 32-bit or a 64-bit >> task respectively. Now a call to relax_compatible_cpus_allowed_ptr() >> will always result in a __sched_setaffinity() call whether there is a >> previous force_compatible_cpus_allowed_ptr() call or not. >> >> In order to fix this regression, a new scheduler flag >> task_struct::cpus_allowed_restricted is now added to track if >> force_compatible_cpus_allowed_ptr() has been called before or not. This >> patch also updates the comments in force_compatible_cpus_allowed_ptr() >> and relax_compatible_cpus_allowed_ptr() and handles their interaction >> with sched_setaffinity(). >> >> This patch also removes the task_user_cpus() helper. In the case of >> relax_compatible_cpus_allowed_ptr(), cpu_possible_mask as user_cpu_ptr >> masking will be performed within __sched_setaffinity() anyway. >> >> Fixes: 8f9ea86fdf99 ("sched: Always preserve the user requested cpumask") >> Reported-by: Will Deacon <will@...nel.org> >> Signed-off-by: Waiman Long <longman@...hat.com> >> --- >> include/linux/sched.h | 3 +++ >> kernel/sched/core.c | 25 +++++++++++++++++-------- >> kernel/sched/sched.h | 8 +------- >> 3 files changed, 21 insertions(+), 15 deletions(-) > So this doesn't even build... > >> diff --git a/kernel/sched/core.c b/kernel/sched/core.c >> index bb1ee6d7bdde..d7bc809c109e 100644 >> --- a/kernel/sched/core.c >> +++ b/kernel/sched/core.c >> @@ -2999,6 +2999,10 @@ static int __set_cpus_allowed_ptr(struct task_struct *p, >> struct rq *rq; >> >> rq = task_rq_lock(p, &rf); >> + >> + if (ctx->flags & SCA_CLR_RESTRICT) >> + p->cpus_allowed_restricted = 0; >> + >> /* >> * Masking should be skipped if SCA_USER or any of the SCA_MIGRATE_* >> * flags are set. >> @@ -3025,8 +3029,8 @@ EXPORT_SYMBOL_GPL(set_cpus_allowed_ptr); >> /* >> * Change a given task's CPU affinity to the intersection of its current >> * affinity mask and @subset_mask, writing the resulting mask to @new_mask. >> - * If user_cpus_ptr is defined, use it as the basis for restricting CPU >> - * affinity or use cpu_online_mask instead. >> + * The cpus_allowed_restricted bit is set to indicate to a later >> + * relax_compatible_cpus_allowed_ptr() call to relax the cpumask. >> * >> * If the resulting mask is empty, leave the affinity unchanged and return >> * -EINVAL. >> @@ -3044,6 +3048,7 @@ static int restrict_cpus_allowed_ptr(struct task_struct *p, >> int err; >> >> rq = task_rq_lock(p, &rf); >> + p->cpus_allowed_restricted = 1; >> >> /* >> * Forcefully restricting the affinity of a deadline task is >> @@ -3055,7 +3060,8 @@ static int restrict_cpus_allowed_ptr(struct task_struct *p, >> goto err_unlock; >> } >> >> - if (!cpumask_and(new_mask, task_user_cpus(p), subset_mask)) { >> + if (p->user_cpu_ptr && >> + !cpumask_and(new_mask, p->user_cpu_ptr, subset_mask)) { > s/user_cpu_ptr/user_cpus_ptr/ > >> err = -EINVAL; >> goto err_unlock; >> } >> @@ -3069,9 +3075,8 @@ static int restrict_cpus_allowed_ptr(struct task_struct *p, >> >> /* >> * Restrict the CPU affinity of task @p so that it is a subset of >> - * task_cpu_possible_mask() and point @p->user_cpus_ptr to a copy of the >> - * old affinity mask. If the resulting mask is empty, we warn and walk >> - * up the cpuset hierarchy until we find a suitable mask. >> + * task_cpu_possible_mask(). If the resulting mask is empty, we warn >> + * and walk up the cpuset hierarchy until we find a suitable mask. >> */ >> void force_compatible_cpus_allowed_ptr(struct task_struct *p) >> { >> @@ -3125,11 +3130,15 @@ __sched_setaffinity(struct task_struct *p, struct affinity_context *ctx); >> void relax_compatible_cpus_allowed_ptr(struct task_struct *p) >> { >> struct affinity_context ac = { >> - .new_mask = task_user_cpus(p), >> - .flags = 0, >> + .new_mask = cpu_possible_mask; > s/;/,/ > > But even with those two things fixed, I'm seeing new failures in my > testing which I think are because restrict_cpus_allowed_ptr() is failing > unexpectedly when called by force_compatible_cpus_allowed_ptr(). > > For example, just running a 32-bit task on an asymmetric system results > in: > > $ ./hello32 > [ 1690.855341] Overriding affinity for process 580 (hello32) to CPUs 2-3 > > That then has knock-on effects such as losing track of the initial affinity > mask and not being able to restore it if the forcefully-affined 32-bit task > exec()s a 64-bit program. I thought I have fixed the build failure. Apparently it is still there. I will fix it. BTW, which arm64 cpus support "allow_mismatched_32bit_el0"? I am trying to see if I can reproduce the issue, but I am not sure if I have any access to the cpus that have this capability. Cheers, Longman
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