Re: [PATCH 04/17] mm: gup: make get_user_pages_fast and
__get_user_pages_fast latency conscious
[Posted October 6, 2014 by corbet]
| From: |
| Linus Torvalds <torvalds-AT-linux-foundation.org> |
| To: |
| Andrea Arcangeli <aarcange-AT-redhat.com> |
| Subject: |
| Re: [PATCH 04/17] mm: gup: make get_user_pages_fast and __get_user_pages_fast latency conscious |
| Date: |
| Fri, 3 Oct 2014 11:23:53 -0700 |
| Message-ID: |
| <CA+55aFyuYRuY9fiJQKL=XJ0-BKhGsZbo1HGkGUOJ6DbbxdA-dQ@mail.gmail.com> |
| Cc: |
| qemu-devel-AT-nongnu.org, KVM list <kvm-AT-vger.kernel.org>, Linux Kernel Mailing List <linux-kernel-AT-vger.kernel.org>, linux-mm <linux-mm-AT-kvack.org>, Linux API <linux-api-AT-vger.kernel.org>, Andres Lagar-Cavilla <andreslc-AT-google.com>, Dave Hansen <dave-AT-sr71.net>, Paolo Bonzini <pbonzini-AT-redhat.com>, Rik van Riel <riel-AT-redhat.com>, Mel Gorman <mgorman-AT-suse.de>, Andy Lutomirski <luto-AT-amacapital.net>, Andrew Morton <akpm-AT-linux-foundation.org>, Sasha Levin <sasha.levin-AT-oracle.com>, Hugh Dickins <hughd-AT-google.com>, Peter Feiner <pfeiner-AT-google.com>, "\\Dr. David Alan Gilbert\\" <dgilbert-AT-redhat.com>, Christopher Covington <cov-AT-codeaurora.org>, Johannes Weiner <hannes-AT-cmpxchg.org>, Android Kernel Team <kernel-team-AT-android.com>, Robert Love <rlove-AT-google.com>, Dmitry Adamushko <dmitry.adamushko-AT-gmail.com>, Neil Brown <neilb-AT-suse.de>, Mike Hommey <mh-AT-glandium.org>, Taras Glek <tg |
| Archive‑link: | |
Article |
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
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