Re: [PATCH v2 2/2] mm: zswap: disable migration while using per-CPU acomp_ctx

[Yosry Ahmed <yosryahmed@google.com>]

On Tue, Jan 7, 2025 at 11:57 PM Barry Song <21cnbao@gmail.com> wrote:
>
> On Wed, Jan 8, 2025 at 6:56 PM Yosry Ahmed <yosryahmed@google.com> wrote:
> >
> > On Tue, Jan 7, 2025 at 9:34 PM Yosry Ahmed <yosryahmed@google.com> wrote:
> > >
> > > On Tue, Jan 7, 2025 at 9:00 PM Chengming Zhou <chengming.zhou@linux.dev> wrote:
> > > >
> > > > On 2025/1/8 12:46, Nhat Pham wrote:
> > > > > On Wed, Jan 8, 2025 at 9:34 AM Yosry Ahmed <yosryahmed@google.com> wrote:
> > > > >>
> > > > >>
> > > > >> Actually, using the mutex to protect against CPU hotunplug is not too
> > > > >> complicated. The following diff is one way to do it (lightly tested).
> > > > >> Johannes, Nhat, any preferences between this patch (disabling
> > > > >> migration) and the following diff?
> > > > >
> > > > > I mean if this works, this over migration diasbling any day? :)
> > > > >
> > > > >>
> > > > >> diff --git a/mm/zswap.c b/mm/zswap.c
> > > > >> index f6316b66fb236..4d6817c679a54 100644
> > > > >> --- a/mm/zswap.c
> > > > >> +++ b/mm/zswap.c
> > > > >> @@ -869,17 +869,40 @@ static int zswap_cpu_comp_dead(unsigned int cpu,
> > > > >> struct hlist_node *node)
> > > > >>          struct zswap_pool *pool = hlist_entry(node, struct zswap_pool, node);
> > > > >>          struct crypto_acomp_ctx *acomp_ctx = per_cpu_ptr(pool->acomp_ctx, cpu);
> > > > >>
> > > > >> +       mutex_lock(&acomp_ctx->mutex);
> > > > >>          if (!IS_ERR_OR_NULL(acomp_ctx)) {
> > > > >>                  if (!IS_ERR_OR_NULL(acomp_ctx->req))
> > > > >>                          acomp_request_free(acomp_ctx->req);
> > > > >> +               acomp_ctx->req = NULL;
> > > > >>                  if (!IS_ERR_OR_NULL(acomp_ctx->acomp))
> > > > >>                          crypto_free_acomp(acomp_ctx->acomp);
> > > > >>                  kfree(acomp_ctx->buffer);
> > > > >>          }
> > > > >> +       mutex_unlock(&acomp_ctx->mutex);
> > > > >>
> > > > >>          return 0;
> > > > >>   }
> > > > >>
> > > > >> +static struct crypto_acomp_ctx *acomp_ctx_get_cpu_locked(
> > > > >> +               struct crypto_acomp_ctx __percpu *acomp_ctx)
> > > > >> +{
> > > > >> +       struct crypto_acomp_ctx *ctx;
> > > > >> +
> > > > >> +       for (;;) {
> > > > >> +               ctx = raw_cpu_ptr(acomp_ctx);
> > > > >> +               mutex_lock(&ctx->mutex);
> > > > >
> > > > > I'm a bit confused. IIUC, ctx is per-cpu right? What's protecting this
> > > > > cpu-local data (including the mutex) from being invalidated under us
> > > > > while we're sleeping and waiting for the mutex?
> > >
> > > Please correct me if I am wrong, but my understanding is that memory
> > > allocated with alloc_percpu() is allocated for each *possible* CPU,
> > > and does not go away when CPUs are offlined. We allocate the per-CPU
> > > crypto_acomp_ctx structs with alloc_percpu() (including the mutex), so
> > > they should not go away with CPU offlining.
> > >
> > > OTOH, we allocate the crypto_acomp_ctx.acompx, crypto_acomp_ctx.req,
> > > and crypto_acomp_ctx.buffer only for online CPUs through the CPU
> > > hotplug notifiers (i.e. zswap_cpu_comp_prepare() and
> > > zswap_cpu_comp_dead()). These are the resources that can go away with
> > > CPU offlining, and what we need to protect about.
> >
> > ..and now that I explain all of this I am wondering if the complexity
> > is warranted in the first place. It goes back all the way to the first
> > zswap commit, so I can't tell the justification for it.
>
> Personally, I would vote for the added complexity, as it avoids the
> potential negative side effects of reverting the scheduler's optimization
> for selecting a suitable CPU for a woken-up task and I have been looking
> for an approach to resolve it by cpuhotplug lock (obviously quite hacky
> and more complex than using this mutex)

Oh, I was not talking about my proposed diff, but the existing logic
that allocates the requests and buffers in the hotplug callbacks
instead of just using alloc_percpu() to allocate them once for each
possible CPU. I was wondering if there are actual setups where this
matters and a significant amount of memory is being saved. Otherwise
we should simplify things and just rip out the hotplug callbacks.

Anyway, for now I will cleanup and send the mutex diff as a new patch.

>
> for (;;)  in acomp_ctx_get_cpu_locked() is a bit tricky but correct and
> really interesting, maybe it needs some comments.
>
> >
> > I am not sure if they are setups that have significantly different
> > numbers of online and possible CPUs. Maybe we should just bite the
> > bullet and just allocate everything with alloc_percpu() and rip out
> > the hotplug callbacks completely.
>
> Thanks
> Barry