linux-kernel - Re: [RFC-PATCH 1/2] mm: Add __GFP_NO

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Date:   Thu, 13 Aug 2020 09:18:02 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Uladzislau Rezki <urezki@...il.com>,
        LKML <linux-kernel@...r.kernel.org>, RCU <rcu@...r.kernel.org>,
        linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Matthew Wilcox <willy@...radead.org>,
        "Theodore Y . Ts'o" <tytso@....edu>,
        Joel Fernandes <joel@...lfernandes.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...ymobile.com>
Subject: Re: [RFC-PATCH 1/2] mm: Add __GFP_NO_LOCKS flag

On Wed 12-08-20 13:38:35, Thomas Gleixner wrote:
> Thomas Gleixner <tglx@...utronix.de> writes:
> > Thomas Gleixner <tglx@...utronix.de> writes:
> >> Michal Hocko <mhocko@...e.com> writes:
> >>> zone->lock should be held for a very limited amount of time.
> >>
> >> Emphasis on should. free_pcppages_bulk() can hold it for quite some time
> >> when a large amount of pages are purged. We surely would have converted
> >> it to a raw lock long time ago otherwise.
> >>
> >> For regular enterprise stuff a few hundred microseconds might qualify as
> >> a limited amount of time. For advanced RT applications that's way beyond
> >> tolerable..
> >
> > Sebastian just tried with zone lock converted to a raw lock and maximum
> > latencies go up by a factor of 7 when putting a bit of stress on the
> > memory subsytem. Just a regular kernel compile kicks them up by a factor
> > of 5. Way out of tolerance.
> >
> > We'll have a look whether it's solely free_pcppages_bulk() and if so we
> > could get away with dropping the lock in the loop.
> 
> So even on !RT and just doing a kernel compile the time spent in
> free_pcppages_bulk() is up to 270 usec.
> 
> It's not only the loop which processes a large pile of pages, part of it
> is caused by lock contention on zone->lock. Dropping the lock after a
> processing a couple of pages does not make it much better if enough CPUs
> are contending on the lock.

OK, this is a bit surprising to me but well, reality sucks sometimes.

I was really hoping for a solution which would allow the allocator to
really do what gfp flags say and if something is nowait then it
shouldn't really block. I believe we need to document this properly.

I will comment on the dedicated gfp flag in reply to other email.

Thanks for trying that out Sebastian and Thomas!
-- 
Michal Hocko
SUSE Labs