All of lore.kernel.org
 help / color / mirror / Atom feed
From: Peter Zijlstra <peterz@infradead.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mateusz Guzik <mjguzik@gmail.com>,
	linux-arch <linux-arch@vger.kernel.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>,
	Michael Ellerman <mpe@ellerman.id.au>,
	linux-fsdevel@vger.kernel.org, viro@zeniv.linux.org.uk,
	Jan Glauber <jan.glauber@gmail.com>,
	tony.luck@intel.com,
	Linux ARM <linux-arm-kernel@lists.infradead.org>,
	linuxppc-dev <linuxppc-dev@lists.ozlabs.org>
Subject: Re: lockref scalability on x86-64 vs cpu_relax
Date: Fri, 13 Jan 2023 11:23:45 +0100	[thread overview]
Message-ID: <Y8ExMQaevecjWjg5@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <CAHk-=wjthxgrLEvgZBUwd35e_mk=dCWKMUEURC6YsX5nWom8kQ@mail.gmail.com>

On Thu, Jan 12, 2023 at 06:13:16PM -0600, Linus Torvalds wrote:
> On Thu, Jan 12, 2023 at 5:36 PM Mateusz Guzik <mjguzik@gmail.com> wrote:
> >
> > To my understanding on said architecture failed cmpxchg still grants you
> > exclusive access to the cacheline, making immediate retry preferable
> > when trying to inc/dec unless a certain value is found.
> 
> I actually suspect that is _always_ the case - this is not like a
> contended spinlock where we want to pause because we're waiting for
> the value to change and become unlocked, this cmpxchg loop is likely
> always better off just retrying with the new value.
> 
> That said, the "likely always better off" is purely about performance.
> 
> So I have this suspicion that the reason Tony added the cpu_relax()
> was simply not about performance, but about other issues, like
> fairness in SMT situations.
> 
> That said, evern from a fairness perspective the cpu_relax() sounds a
> bit odd and unlikely - we're literally yielding when we lost a race,
> so it hurts the _loser_, not the winner, and thus might make fairness
> worse too.

I've been writing cmpxchg loops that have strict termination conditions
without cpu_relax() in them for a while now.

For example, the x86 atomic_fetch_and() implementation looks like so:

static __always_inline int arch_atomic_fetch_and(int i, atomic_t *v)
{
	int val = arch_atomic_read(v);

	do { } while (!arch_atomic_try_cmpxchg(v, &val, val & i));

	return val;
}

And I did that because of the exact same argument you had above, it
needs to do the op anyway, waiting between failed attempts will only
increase the chance it will fail again.

WARNING: multiple messages have this Message-ID (diff)
From: Peter Zijlstra <peterz@infradead.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: linux-arch <linux-arch@vger.kernel.org>,
	Mateusz Guzik <mjguzik@gmail.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
	tony.luck@intel.com, viro@zeniv.linux.org.uk,
	linux-fsdevel@vger.kernel.org,
	Jan Glauber <jan.glauber@gmail.com>,
	Will Deacon <will@kernel.org>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>
Subject: Re: lockref scalability on x86-64 vs cpu_relax
Date: Fri, 13 Jan 2023 11:23:45 +0100	[thread overview]
Message-ID: <Y8ExMQaevecjWjg5@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <CAHk-=wjthxgrLEvgZBUwd35e_mk=dCWKMUEURC6YsX5nWom8kQ@mail.gmail.com>

On Thu, Jan 12, 2023 at 06:13:16PM -0600, Linus Torvalds wrote:
> On Thu, Jan 12, 2023 at 5:36 PM Mateusz Guzik <mjguzik@gmail.com> wrote:
> >
> > To my understanding on said architecture failed cmpxchg still grants you
> > exclusive access to the cacheline, making immediate retry preferable
> > when trying to inc/dec unless a certain value is found.
> 
> I actually suspect that is _always_ the case - this is not like a
> contended spinlock where we want to pause because we're waiting for
> the value to change and become unlocked, this cmpxchg loop is likely
> always better off just retrying with the new value.
> 
> That said, the "likely always better off" is purely about performance.
> 
> So I have this suspicion that the reason Tony added the cpu_relax()
> was simply not about performance, but about other issues, like
> fairness in SMT situations.
> 
> That said, evern from a fairness perspective the cpu_relax() sounds a
> bit odd and unlikely - we're literally yielding when we lost a race,
> so it hurts the _loser_, not the winner, and thus might make fairness
> worse too.

I've been writing cmpxchg loops that have strict termination conditions
without cpu_relax() in them for a while now.

For example, the x86 atomic_fetch_and() implementation looks like so:

static __always_inline int arch_atomic_fetch_and(int i, atomic_t *v)
{
	int val = arch_atomic_read(v);

	do { } while (!arch_atomic_try_cmpxchg(v, &val, val & i));

	return val;
}

And I did that because of the exact same argument you had above, it
needs to do the op anyway, waiting between failed attempts will only
increase the chance it will fail again.

WARNING: multiple messages have this Message-ID (diff)
From: Peter Zijlstra <peterz@infradead.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mateusz Guzik <mjguzik@gmail.com>,
	linux-arch <linux-arch@vger.kernel.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>,
	Michael Ellerman <mpe@ellerman.id.au>,
	linux-fsdevel@vger.kernel.org, viro@zeniv.linux.org.uk,
	Jan Glauber <jan.glauber@gmail.com>,
	tony.luck@intel.com,
	Linux ARM <linux-arm-kernel@lists.infradead.org>,
	linuxppc-dev <linuxppc-dev@lists.ozlabs.org>
Subject: Re: lockref scalability on x86-64 vs cpu_relax
Date: Fri, 13 Jan 2023 11:23:45 +0100	[thread overview]
Message-ID: <Y8ExMQaevecjWjg5@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <CAHk-=wjthxgrLEvgZBUwd35e_mk=dCWKMUEURC6YsX5nWom8kQ@mail.gmail.com>

On Thu, Jan 12, 2023 at 06:13:16PM -0600, Linus Torvalds wrote:
> On Thu, Jan 12, 2023 at 5:36 PM Mateusz Guzik <mjguzik@gmail.com> wrote:
> >
> > To my understanding on said architecture failed cmpxchg still grants you
> > exclusive access to the cacheline, making immediate retry preferable
> > when trying to inc/dec unless a certain value is found.
> 
> I actually suspect that is _always_ the case - this is not like a
> contended spinlock where we want to pause because we're waiting for
> the value to change and become unlocked, this cmpxchg loop is likely
> always better off just retrying with the new value.
> 
> That said, the "likely always better off" is purely about performance.
> 
> So I have this suspicion that the reason Tony added the cpu_relax()
> was simply not about performance, but about other issues, like
> fairness in SMT situations.
> 
> That said, evern from a fairness perspective the cpu_relax() sounds a
> bit odd and unlikely - we're literally yielding when we lost a race,
> so it hurts the _loser_, not the winner, and thus might make fairness
> worse too.

I've been writing cmpxchg loops that have strict termination conditions
without cpu_relax() in them for a while now.

For example, the x86 atomic_fetch_and() implementation looks like so:

static __always_inline int arch_atomic_fetch_and(int i, atomic_t *v)
{
	int val = arch_atomic_read(v);

	do { } while (!arch_atomic_try_cmpxchg(v, &val, val & i));

	return val;
}

And I did that because of the exact same argument you had above, it
needs to do the op anyway, waiting between failed attempts will only
increase the chance it will fail again.

_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel

  parent reply	other threads:[~2023-01-13 10:25 UTC|newest]

Thread overview: 108+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2023-01-12 23:36 lockref scalability on x86-64 vs cpu_relax Mateusz Guzik
2023-01-13  0:13 ` Linus Torvalds
2023-01-13  0:13   ` Linus Torvalds
2023-01-13  0:13   ` Linus Torvalds
2023-01-13  0:30   ` Luck, Tony
2023-01-13  0:30     ` Luck, Tony
2023-01-13  0:30     ` Luck, Tony
2023-01-13  0:45     ` Linus Torvalds
2023-01-13  0:45       ` Linus Torvalds
2023-01-13  0:45       ` Linus Torvalds
2023-01-13  7:55     ` ia64 removal (was: Re: lockref scalability on x86-64 vs cpu_relax) Ard Biesheuvel
2023-01-13  7:55       ` Ard Biesheuvel
2023-01-13  7:55       ` Ard Biesheuvel
2023-01-13 16:17       ` Luck, Tony
2023-01-13 16:17         ` Luck, Tony
2023-01-13 16:17         ` Luck, Tony
2023-01-13 20:49       ` Jessica Clarke
2023-01-13 20:49         ` Jessica Clarke
2023-01-13 20:49         ` Jessica Clarke
2023-01-13 21:03         ` Luck, Tony
2023-01-13 21:03           ` Luck, Tony
2023-01-13 21:03           ` Luck, Tony
2023-01-13 21:04           ` Jessica Clarke
2023-01-13 21:04             ` Jessica Clarke
2023-01-13 21:04             ` Jessica Clarke
2023-01-13 21:05       ` John Paul Adrian Glaubitz
2023-01-13 21:05         ` John Paul Adrian Glaubitz
2023-01-13 21:05         ` John Paul Adrian Glaubitz
2023-01-13 23:25         ` Ard Biesheuvel
2023-01-13 23:25           ` Ard Biesheuvel
2023-01-13 23:25           ` Ard Biesheuvel
2023-01-14 11:24           ` Sedat Dilek
2023-01-14 11:24             ` Sedat Dilek
2023-01-14 11:24             ` Sedat Dilek
2023-01-14 11:28             ` Sedat Dilek
2023-01-14 11:28               ` Sedat Dilek
2023-01-14 11:28               ` Sedat Dilek
2023-01-15  0:27               ` Matthew Wilcox
2023-01-15  0:27                 ` Matthew Wilcox
2023-01-15  0:27                 ` Matthew Wilcox
2023-01-15 12:04                 ` Sedat Dilek
2023-01-15 12:04                   ` Sedat Dilek
2023-01-15 12:04                   ` Sedat Dilek
2023-01-16  9:42                   ` John Paul Adrian Glaubitz
2023-01-16  9:42                     ` John Paul Adrian Glaubitz
2023-01-16  9:42                     ` John Paul Adrian Glaubitz
2023-01-16  9:41                 ` John Paul Adrian Glaubitz
2023-01-16  9:41                   ` John Paul Adrian Glaubitz
2023-01-16  9:41                   ` John Paul Adrian Glaubitz
2023-01-16 13:28                   ` Matthew Wilcox
2023-01-16 13:28                     ` Matthew Wilcox
2023-01-16 13:28                     ` Matthew Wilcox
2023-01-16  9:40               ` John Paul Adrian Glaubitz
2023-01-16  9:40                 ` John Paul Adrian Glaubitz
2023-01-16  9:40                 ` John Paul Adrian Glaubitz
2023-01-16  9:37             ` John Paul Adrian Glaubitz
2023-01-16  9:37               ` John Paul Adrian Glaubitz
2023-01-16  9:37               ` John Paul Adrian Glaubitz
2023-01-16  9:32           ` John Paul Adrian Glaubitz
2023-01-16  9:32             ` John Paul Adrian Glaubitz
2023-01-16  9:32             ` John Paul Adrian Glaubitz
2023-01-16 10:09             ` Ard Biesheuvel
2023-01-16 10:09               ` Ard Biesheuvel
2023-01-16 10:09               ` Ard Biesheuvel
2023-01-13  1:12   ` lockref scalability on x86-64 vs cpu_relax Mateusz Guzik
2023-01-13  1:12     ` Mateusz Guzik
2023-01-13  1:12     ` Mateusz Guzik
2023-01-13  4:08     ` Linus Torvalds
2023-01-13  4:08       ` Linus Torvalds
2023-01-13  4:08       ` Linus Torvalds
2023-01-13  9:46     ` Will Deacon
2023-01-13  9:46       ` Will Deacon
2023-01-13  9:46       ` Will Deacon
2023-01-13  3:20   ` Nicholas Piggin
2023-01-13  3:20     ` Nicholas Piggin
2023-01-13  3:20     ` Nicholas Piggin
2023-01-13  4:15     ` Linus Torvalds
2023-01-13  4:15       ` Linus Torvalds
2023-01-13  4:15       ` Linus Torvalds
2023-01-13  5:36       ` Nicholas Piggin
2023-01-13  5:36         ` Nicholas Piggin
2023-01-13  5:36         ` Nicholas Piggin
2023-01-16 14:08     ` Memory transaction instructions David Howells
2023-01-16 14:08       ` David Howells
2023-01-16 15:09       ` Matthew Wilcox
2023-01-16 15:09         ` Matthew Wilcox
2023-01-16 15:09         ` Matthew Wilcox
2023-01-16 16:59       ` Linus Torvalds
2023-01-16 16:59         ` Linus Torvalds
2023-01-16 16:59         ` Linus Torvalds
2023-01-18  9:05       ` David Howells
2023-01-18  9:05         ` David Howells
2023-01-18  9:05         ` David Howells
2023-01-19  1:41         ` Nicholas Piggin
2023-01-19  1:41           ` Nicholas Piggin
2023-01-19  1:41           ` Nicholas Piggin
2023-01-13 10:23   ` Peter Zijlstra [this message]
2023-01-13 10:23     ` lockref scalability on x86-64 vs cpu_relax Peter Zijlstra
2023-01-13 10:23     ` Peter Zijlstra
2023-01-13 18:44   ` [PATCH] lockref: stop doing cpu_relax in the cmpxchg loop Mateusz Guzik
2023-01-13 18:44     ` Mateusz Guzik
2023-01-13 18:44     ` Mateusz Guzik
2023-01-13 21:47     ` Luck, Tony
2023-01-13 21:47       ` Luck, Tony
2023-01-13 21:47       ` Luck, Tony
2023-01-13 23:31       ` Linus Torvalds
2023-01-13 23:31         ` Linus Torvalds
2023-01-13 23:31         ` Linus Torvalds

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=Y8ExMQaevecjWjg5@hirez.programming.kicks-ass.net \
    --to=peterz@infradead.org \
    --cc=catalin.marinas@arm.com \
    --cc=jan.glauber@gmail.com \
    --cc=linux-arch@vger.kernel.org \
    --cc=linux-arm-kernel@lists.infradead.org \
    --cc=linux-fsdevel@vger.kernel.org \
    --cc=linuxppc-dev@lists.ozlabs.org \
    --cc=mjguzik@gmail.com \
    --cc=mpe@ellerman.id.au \
    --cc=tony.luck@intel.com \
    --cc=torvalds@linux-foundation.org \
    --cc=viro@zeniv.linux.org.uk \
    --cc=will@kernel.org \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body.
This is an external index of several public inboxes,
see mirroring instructions on how to clone and mirror
all data and code used by this external index.