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Message-ID: <87k0y56wc1.fsf@nanos.tec.linutronix.de>
Date: Tue, 11 Aug 2020 17:22:54 +0200
From: Thomas Gleixner <tglx@...utronix.de>
To: Michal Hocko <mhocko@...e.com>, Uladzislau Rezki <urezki@...il.com>
Cc: 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
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.
Thanks,
tglx
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