linux-kernel - Re: [RESEND][PATCH v2] mm: don't call lru draining in the nested lru_cache

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Date:   Fri, 21 Jan 2022 13:56:31 -0800
From:   Minchan Kim <minchan@...nel.org>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        David Hildenbrand <david@...hat.com>,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        John Dias <joaodias@...gle.com>
Subject: Re: [RESEND][PATCH v2] mm: don't call lru draining in the nested
 lru_cache_disable

On Fri, Jan 21, 2022 at 10:59:32AM +0100, Michal Hocko wrote:
> On Thu 20-01-22 13:07:55, Minchan Kim wrote:
> > On Thu, Jan 20, 2022 at 09:24:22AM +0100, Michal Hocko wrote:
> > > On Wed 19-01-22 20:25:54, Minchan Kim wrote:
> > > > On Wed, Jan 19, 2022 at 10:20:22AM +0100, Michal Hocko wrote:
> > > [...]
> > > > > What does prevent you from calling lru_cache_{disable,enable} this way
> > > > > with the existing implementation? AFAICS calls can be nested just fine.
> > > > > Or am I missing something?
> > > > 
> > > > It just increases more IPI calls since we drain the lru cache
> > > > both upper layer and lower layer. That's I'd like to avoid
> > > > in this patch. Just disable lru cache one time for entire
> > > > allocation path.
> > > 
> > > I do not follow. Once you call lru_cache_disable at the higher level
> > > then no new pages are going to be added to the pcp caches. At the same
> > > time existing caches are flushed so the inner lru_cache_disable will not
> > > trigger any new IPIs.
> > 
> > lru_cache_disable calls __lru_add_drain_all with force_all_cpus
> > unconditionally so keep calling the IPI.
> 
> OK, this is something I have missed. Why cannot we remove the force_all
> mode for lru_disable_count>0 when there are no pcp caches populated?

Couldn't gaurantee whether the IPI is finished with only atomic counter.

CPU 0                               CPU 1
lru_cache_disable                   lru_cache_disable
  ret = atomic_inc_return
                                    
                                   ret = atomic_inc_return
  lru_add_drain_all(ret == 1);     lru_add_drain_all(ret == 1)
    IPI ongoing                    skip IPI
                                   alloc_contig_range
                                   fail
    ..
    ..

   IPI done