Re: [PATCH 0/6] Switch arm64 over to qrwlock

From: Jan Glauber <jan.glauber@gmail.com>
To: Will Deacon <will.deacon@arm.com>
Cc: linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org,
	Jeremy Linton <Jeremy.Linton@arm.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@redhat.com>,
	longman@redhat.com, boqun.feng@gmail.com,
	paulmck@linux.vnet.ibm.com
Subject: Re: [PATCH 0/6] Switch arm64 over to qrwlock
Date: Tue, 10 Oct 2017 17:17:26 +0200	[thread overview]
Message-ID: <CAEiAFz3k=jGg2Ct2ucUGq+FsZeSo4C_U12FyQmQKy2tA6doSwQ@mail.gmail.com> (raw)

2017-10-05 14:54 GMT+02:00 Will Deacon <will.deacon@arm.com>:
>
> Hi all,
>
> This patch series reworks bits of the qrwlock code that it can be used
> to replace the asm rwlocks currently implemented for arm64. The structure
> of the series is:
>
>   Patches 1-3   : Work WFE into qrwlock using atomic_cond_read_acquire so
>                   we can avoid busy-waiting.
>
>   Patch 4       : Enable qrwlocks for arm64
>
>   Patch 5-6     : Ensure writer slowpath fairness. This has a potential
>                   performance impact on the writer unlock path, so I've
>                   kept them at the end.
>
> The patches apply on top of my other locking cleanups:
>
>   http://lkml.kernel.org/r/1507055129-12300-1-git-send-email-will.deacon@arm.com
>
> although the conflict with mainline is trivial to resolve without those.
> The full stack is also pushed here:
>
>   git://git.kernel.org/pub/scm/linux/kernel/git/will/linux.git qrwlock
>
> All comments (particularly related to testing and performance) welcome!

Hi Will,

run your patches on ThunderX2.

Old RW locks:

insmod ./locktorture.ko nwriters_stress=56 nreaders_stress=224
torture_type="rw_lock_irq" stat_interval=2

[  558.700042] Writes:  Total: 10127  Max/Min: 0/0   Fail: 0
[  558.700054] Reads :  Total: 764714  Max/Min: 0/0   Fail: 0
[  561.797011] Writes:  Total: 11288  Max/Min: 0/0   Fail: 0
[  561.802518] Reads :  Total: 2104452  Max/Min: 0/0   Fail: 0
[  565.844219] Writes:  Total: 11512  Max/Min: 0/0   Fail: 0
[  565.849710] Reads :  Total: 4277492  Max/Min: 0/0   Fail: 0

Queued RW locks:

[  221.491207] Writes:  Total: 57318  Max/Min: 0/0   Fail: 0
[  221.491219] Reads :  Total: 382979  Max/Min: 0/0   Fail: 0
[  223.507065] Writes:  Total: 83490  Max/Min: 0/0   Fail: 0
[  223.512611] Reads :  Total: 684848  Max/Min: 0/0   Fail: 0
[  225.522937] Writes:  Total: 110012  Max/Min: 0/0   Fail: 0
[  225.528511] Reads :  Total: 968826  Max/Min: 0/0   Fail: 0

So readers are still preferred over writers, but results are _way_
better. Also, with the old implementation
above test hung the machine which does not happen with the queued variant.

If you want you can add:
Tested-by: Jan Glauber <jglauber@cavium.com>

--Jan

> Cheers,
>
> Will
>
> --->8
>
> Will Deacon (6):
>   kernel/locking: Use struct qrwlock instead of struct __qrwlock
>   locking/atomic: Add atomic_cond_read_acquire
>   kernel/locking: Use atomic_cond_read_acquire when spinning in qrwlock
>   arm64: locking: Move rwlock implementation over to qrwlocks
>   kernel/locking: Prevent slowpath writers getting held up by fastpath
>   kernel/locking: Remove unused union members from struct qrwlock
>
>  arch/arm64/Kconfig                      |  17 ++++
>  arch/arm64/include/asm/Kbuild           |   1 +
>  arch/arm64/include/asm/spinlock.h       | 164 +-------------------------------
>  arch/arm64/include/asm/spinlock_types.h |   6 +-
>  include/asm-generic/atomic-long.h       |   3 +
>  include/asm-generic/qrwlock.h           |  14 +--
>  include/asm-generic/qrwlock_types.h     |   2 +-
>  include/linux/atomic.h                  |   4 +
>  kernel/locking/qrwlock.c                |  83 +++-------------
>  9 files changed, 43 insertions(+), 251 deletions(-)
>
> --
> 2.1.4
>