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Date: Fri, 3 Oct 2014 11:23:53 -0700
Message-ID: <CA+55aFyuYRuY9fiJQKL=XJ0-BKhGsZbo1HGkGUOJ6DbbxdA-dQ@mail.gmail.com>
From: Linus Torvalds <torvalds@linux-foundation.org>
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Subject: Re: [Qemu-devel] [PATCH 04/17] mm: gup: make get_user_pages_fast
 and __get_user_pages_fast latency conscious
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To: Andrea Arcangeli <aarcange@redhat.com>
Cc: Robert Love <rlove@google.com>, Dave Hansen <dave@sr71.net>, Jan Kara <jack@suse.cz>, KVM list <kvm@vger.kernel.org>, Neil Brown <neilb@suse.de>, Stefan Hajnoczi <stefanha@gmail.com>, qemu-devel@nongnu.org, linux-mm <linux-mm@kvack.org>, KOSAKI Motohiro <kosaki.motohiro@gmail.com>, Michel Lespinasse <walken@google.com>, Taras Glek <tglek@mozilla.com>, Andrew Jones <drjones@redhat.com>, Juan Quintela <quintela@redhat.com>, Hugh Dickins <hughd@google.com>, Isaku Yamahata <yamahata@valinux.co.jp>, Mel Gorman <mgorman@suse.de>, Sasha Levin <sasha.levin@oracle.com>, Android Kernel Team <kernel-team@android.com>, "\\Dr. David Alan Gilbert\\" <dgilbert@redhat.com>, "Huangpeng (Peter)" <peter.huangpeng@huawei.com>, Andres Lagar-Cavilla <andreslc@google.com>, Christopher Covington <cov@codeaurora.org>, Anthony Liguori <anthony@codemonkey.ws>, Mike Hommey <mh@glandium.org>, Keith Packard <keithp@keithp.com>, Wenchao Xia <wenchaoqemu@gmail.com>, Linux API <linux-api@vger.kernel.org>, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, Andy Lutomirski <luto@amacapital.net>, Minchan Kim <minchan@kernel.org>, Dmitry Adamushko <dmitry.adamushko@gmail.com>, Johannes Weiner <hannes@cmpxchg.org>, Paolo Bonzini <pbonzini@redhat.com>, Andrew Morton <akpm@linux-foundation.org>, Peter Feiner <pfeiner@google.com>

On Fri, Oct 3, 2014 at 10:07 AM, Andrea Arcangeli <aarcange@redhat.com> wrote:
> This teaches gup_fast and __gup_fast to re-enable irqs and
> cond_resched() if possible every BATCH_PAGES.

This is disgusting.

Many (most?) __gup_fast() users just want a single page, and the
stupid overhead of the multi-page version is already unnecessary.
This just makes things much worse.

Quite frankly, we should make a single-page version of __gup_fast(),
and convert existign users to use that. After that, the few multi-page
users could have this extra latency control stuff.

And yes, the single-page version of get_user_pages_fast() is actually
latency-critical. shared futexes hit it hard, and yes, I've seen this
in profiles.

                  Linus