Messages in this thread | | | Date | Tue, 17 Jan 2023 13:13:31 -0500 | Subject | Re: [PATCH v10 2/5] sched: Use user_cpus_ptr for saving user provided cpumask in sched_setaffinity() | From | Waiman Long <> |
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On 1/17/23 11:08, Will Deacon wrote: > 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.
IOW, relax_compatible_cpus_allowed_ptr() can be called without a previous force_compatible_cpus_allowed_ptr(). Right?
A possible optimization in this case is to add a bit flag in the task_struct to indicate a previous call to force_compatible_cpus_allowed_ptr(). Without that flag set, relax_compatible_cpus_allowed_ptr() can return immediately.
> > 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' ... The flag bit will work like the user_mask check here. > >> - >> - 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).
CPU hotplug should update the cpumask of existing running application as allowed by cpuset.
> > 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?
The problem here is that force_compatible_cpus_allowed_ptr() can be called without a matching relax_compatible_cpus_allowed_ptr() at the end. So we may end up artificially restrict the number of cpus that can be used when running a 64-bit binary.
What do you think about the idea of having a bit flag to track that?
Cheers, Longman
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