linux-kernel - Re: [PATCH 1/2] sched/wait: Break up long wake list walk

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Date:   Wed, 16 Aug 2017 18:22:57 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Andi Kleen <ak@...ux.intel.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...e.hu>, Kan Liang <kan.liang@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>, Jan Kara <jack@...e.cz>,
        linux-mm <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] sched/wait: Break up long wake list walk

Linus Torvalds <torvalds@...ux-foundation.org> writes:

> On Tue, Aug 15, 2017 at 3:57 PM, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
>>
>> Oh, and the page wait-queue really needs that key argument too, which
>> is another thing that swait queue code got rid of in the name of
>> simplicity.
>
> Actually, it gets worse.
>
> Because the page wait queues are hashed, it's not an all-or-nothing
> thing even for the non-exclusive cases, and it's not a "wake up first
> entry" for the exclusive case. Both have to be conditional on the wait
> entry actually matching the page and bit in question.
>
> So no way to use swait, or any of the lockless queuing code in general
> (so we can't do some clever private wait-list using llist.h either).
>
> End result: it looks like you fairly fundamentally do need to use a
> lock over the whole list traversal (like the standard wait-queues),
> and then add a cursor entry like Tim's patch if dropping the lock in
> the middle.
>
> Anyway, looking at the old code, we *used* to limit the page wait hash
> table to 4k entries, and we used to have one hash table per memory
> zone.
>
> The per-zone thing didn't work at all for the generic bit-waitqueues,
> because of how people used them on virtual addresses on the stack.
>
> But it *could* work for the page waitqueues, which are now a totally
> separate entity, and is obviously always physically addressed (since
> the indexing is by "struct page" pointer), and doesn't have that
> issue.
>
> So I guess we could re-introduce the notion of per-zone page waitqueue
> hash tables. It was disgusting to allocate and free though (and hooked
> into the memory hotplug code).
>
> So I'd still hope that we can instead just have one larger hash table,
> and that is sufficient for the problem.

If increasing the hash table size fixes the problem I am wondering if
rhash tables might be the proper solution to this problem.  They start
out small and then grow as needed.

Eric