Messages in this thread Patch in this message | | | Date | Tue, 17 Jan 2023 16:08:26 +0000 | From | Will Deacon <> | Subject | Re: [PATCH v10 2/5] sched: Use user_cpus_ptr for saving user provided cpumask in sched_setaffinity() |
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Hi Waiman,
On Thu, Sep 22, 2022 at 02:00:38PM -0400, Waiman Long wrote: > The user_cpus_ptr field is added by commit b90ca8badbd1 ("sched: > Introduce task_struct::user_cpus_ptr to track requested affinity"). It > is currently used only by arm64 arch due to possible asymmetric CPU > setup. This patch extends its usage to save user provided cpumask > when sched_setaffinity() is called for all arches. With this patch > applied, user_cpus_ptr, once allocated after a successful call to > sched_setaffinity(), will only be freed when the task exits. > > Since user_cpus_ptr is supposed to be used for "requested > affinity", there is actually no point to save current cpu affinity in > restrict_cpus_allowed_ptr() if sched_setaffinity() has never been called. > Modify the logic to set user_cpus_ptr only in sched_setaffinity() and use > it in restrict_cpus_allowed_ptr() and relax_compatible_cpus_allowed_ptr() > if defined but not changing it. > > This will be some changes in behavior for arm64 systems with asymmetric > CPUs in some corner cases. For instance, if sched_setaffinity() > has never been called and there is a cpuset change before > relax_compatible_cpus_allowed_ptr() is called, its subsequent call will > follow what the cpuset allows but not what the previous cpu affinity > setting allows. > > Signed-off-by: Waiman Long <longman@redhat.com> > --- > kernel/sched/core.c | 82 ++++++++++++++++++++------------------------ > kernel/sched/sched.h | 7 ++++ > 2 files changed, 44 insertions(+), 45 deletions(-)
We've tracked this down as the cause of an arm64 regression in Android and I've reproduced the issue with mainline.
Basically, if an arm64 system is booted with "allow_mismatched_32bit_el0" on the command-line, then the arch code will (amongst other things) call force_compatible_cpus_allowed_ptr() and relax_compatible_cpus_allowed_ptr() when exec()'ing a 32-bit or a 64-bit task respectively.
If you consider a system where everything is 64-bit but the cmdline option above is present, then the call to relax_compatible_cpus_allowed_ptr() isn't expected to do anything in this case, and the old code made sure of that:
> @@ -3055,30 +3032,21 @@ __sched_setaffinity(struct task_struct *p, const struct cpumask *mask); > > /* > * Restore the affinity of a task @p which was previously restricted by a > - * call to force_compatible_cpus_allowed_ptr(). This will clear (and free) > - * @p->user_cpus_ptr. > + * call to force_compatible_cpus_allowed_ptr(). > * > * It is the caller's responsibility to serialise this with any calls to > * force_compatible_cpus_allowed_ptr(@p). > */ > void relax_compatible_cpus_allowed_ptr(struct task_struct *p) > { > - struct cpumask *user_mask = p->user_cpus_ptr; > - unsigned long flags; > + int ret; > > /* > - * Try to restore the old affinity mask. If this fails, then > - * we free the mask explicitly to avoid it being inherited across > - * a subsequent fork(). > + * Try to restore the old affinity mask with __sched_setaffinity(). > + * Cpuset masking will be done there too. > */ > - if (!user_mask || !__sched_setaffinity(p, user_mask)) > - return;
... since it returned early here if '!user_mask' ...
> - > - raw_spin_lock_irqsave(&p->pi_lock, flags); > - user_mask = clear_user_cpus_ptr(p); > - raw_spin_unlock_irqrestore(&p->pi_lock, flags); > - > - kfree(user_mask); > + ret = __sched_setaffinity(p, task_user_cpus(p)); > + WARN_ON_ONCE(ret);
... however, now we end up going down into __sched_setaffinity() with task_user_cpus(p) giving us the 'cpu_possible_mask'! This can lead to a mixture of WARN_ON()s and incorrect affinity masks (for example, a newly exec'd task ends up with the affinity mask of the online CPUs at the point of exec() and is unable to run on anything onlined later).
I've had a crack at fixing the code above to restore the old behaviour, and it seems to work for my basic tests (still pending confirmation from others):
diff --git a/kernel/sched/core.c b/kernel/sched/core.c index bb1ee6d7bdde..0d4a11384648 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -3125,17 +3125,16 @@ __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), + .new_mask = p->user_cpus_ptr, .flags = 0, }; - int ret; /* * Try to restore the old affinity mask with __sched_setaffinity(). * Cpuset masking will be done there too. */ - ret = __sched_setaffinity(p, &ac); - WARN_ON_ONCE(ret); + if (ac.new_mask) + WARN_ON_ONCE(__sched_setaffinity(p, &ac)); } void set_task_cpu(struct task_struct *p, unsigned int new_cpu)
With this change, task_user_cpus() is only used by restrict_cpus_allowed_ptr() so I'd be inclined to remove it altogether tbh.
What do you think?
Will
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