Re: [PATCH] arm64/mm: Add memory barrier for mm_cid

[levi.yun]

Hi Mathieu!

On 05/03/2024 20:01, Mathieu Desnoyers wrote:
> On 2024-03-05 09:53, levi.yun wrote:
> > Currently arm64's switch_mm() doesn't always have an smp_mb()
> > which the core scheduler code has depended upon since commit:
> >
> >      commit 223baf9d17f25 ("sched: Fix performance regression 
> > introduced by mm_cid")
> >
> > If switch_mm() doesn't call smp_mb(), sched_mm_cid_remote_clear()
> > can unset the activly used cid when it fails to observe active task 
> > after it
> > sets lazy_put.
> >
> > By adding an smp_mb() in arm64's check_and_switch_context(),
> > Guarantee to observe active task after sched_mm_cid_remote_clear()
> > success to set lazy_put.
>
> This comment from the original implementation of membarrier
> MEMBARRIER_CMD_PRIVATE_EXPEDITED states that the original need from
> membarrier was to have a full barrier between storing to rq->curr and
> return to userspace:
>
> commit 22e4ebb9758 ("membarrier: Provide expedited private command")
>
> commit message:
>
>     * Our TSO archs can do RELEASE without being a full barrier. Look at
>       x86 spin_unlock() being a regular STORE for example.  But for those
>       archs, all atomics imply smp_mb and all of them have atomic ops in
>       switch_mm() for mm_cpumask(), and on x86 the CR3 load acts as a 
> full
>       barrier.
>         * From all weakly ordered machines, only ARM64 and PPC can do 
> RELEASE,
>       the rest does indeed do smp_mb(), so there the spin_unlock() is 
> a full
>       barrier and we're good.
>         * ARM64 has a very heavy barrier in switch_to(), which suffices.
>         * PPC just removed its barrier from switch_to(), but appears 
> to be
>       talking about adding something to switch_mm(). So add a
>       smp_mb__after_unlock_lock() for now, until this is settled on 
> the PPC
>       side.
>
> associated code:
>
> +               /*
> +                * The membarrier system call requires each architecture
> +                * to have a full memory barrier after updating
> +                * rq->curr, before returning to user-space. For TSO
> +                * (e.g. x86), the architecture must provide its own
> +                * barrier in switch_mm(). For weakly ordered machines
> +                * for which spin_unlock() acts as a full memory
> +                * barrier, finish_lock_switch() in common code takes
> +                * care of this barrier. For weakly ordered machines for
> +                * which spin_unlock() acts as a RELEASE barrier (only
> +                * arm64 and PowerPC), arm64 has a full barrier in
> +                * switch_to(), and PowerPC has
> +                * smp_mb__after_unlock_lock() before
> +                * finish_lock_switch().
> +                */
>
> Which got updated to this by
>
> commit 306e060435d ("membarrier: Document scheduler barrier 
> requirements")
>
>                 /*
>                  * The membarrier system call requires each architecture
>                  * to have a full memory barrier after updating
> +                * rq->curr, before returning to user-space.
> +                *
> +                * Here are the schemes providing that barrier on the
> +                * various architectures:
> +                * - mm ? switch_mm() : mmdrop() for x86, s390, sparc, 
> PowerPC.
> +                *   switch_mm() rely on membarrier_arch_switch_mm() 
> on PowerPC.
> +                * - finish_lock_switch() for weakly-ordered
> +                *   architectures where spin_unlock is a full barrier,
> +                * - switch_to() for arm64 (weakly-ordered, spin_unlock
> +                *   is a RELEASE barrier),
>                  */
>
> However, rseq mm_cid has stricter requirements: the barrier needs to be
> issued between store to rq->curr and switch_mm_cid(), which happens
> earlier than:
>
> - spin_unlock(),
> - switch_to().
>
> So it's fine when the architecture switch_mm happens to have that barrier
> already, but less so when the architecture only provides the full barrier
> in switch_to() or spin_unlock().
>
> The issue is therefore not specific to arm64, it's actually a bug in the
> rseq switch_mm_cid() implementation. All architectures that don't have
> memory barriers in switch_mm(), but rather have the full barrier 
> either in
> finish_lock_switch() or switch_to() have them too late for the needs of
> switch_mm_cid().

Thanks for the great detail explain!


> I would recommend one of three approaches here:
>
> A) Add smp_mb() in switch_mm_cid() for all architectures that lack that
>    barrier in switch_mm().
>
> B) Figure out if we can move switch_mm_cid() further down in the 
> scheduler
>    without breaking anything (within switch_to(), at the very end of
>    finish_lock_switch() for instance). I'm not sure we can do that though
>    because switch_mm_cid() touches the "prev" which is tricky after
>    switch_to().
>
> C) Add barriers in switch_mm() within all architectures that are 
> missing it.
>
> Thoughts ?
IMHO, A) is look good to me.

Because, In case of B), If you assume spin_unlock() for rq->lock has 
full memory barrier,
I'm not sure about the architecture which using queued_spin_unlock().

When I see the queued_spin_unlock()'s implementation, It implements 
using smp_store_relasse().
But, when we see the memory_barrier.txt describing MULTICOPY ATOMICITY,
If smp_mb__after_atomic() is implemented with smp_mb(), There might fail 
to observe.

Am I wrong?

Many thanks!