From: Al Viro <viro@zeniv.linux.org.uk>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: kernel test robot <oliver.sang@intel.com>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@kernel.org>, Borislav Petkov <bp@alien8.de>,
Peter Zijlstra <peterz@infradead.org>,
LKML <linux-kernel@vger.kernel.org>,
lkp@lists.01.org, kernel test robot <lkp@intel.com>,
"Huang, Ying" <ying.huang@intel.com>,
Feng Tang <feng.tang@intel.com>,
zhengjun.xing@intel.com
Subject: Re: [x86] d55564cfc2: will-it-scale.per_thread_ops -5.8% regression
Date: Thu, 7 Jan 2021 18:55:59 +0000 [thread overview]
Message-ID: <20210107185559.GI3579531@ZenIV.linux.org.uk> (raw)
In-Reply-To: <20210107184058.GH3579531@ZenIV.linux.org.uk>
On Thu, Jan 07, 2021 at 06:40:58PM +0000, Al Viro wrote:
> do_sys_poll(): do the wholesale copyout
>
> Don't bother with patching up just one field - 16 bits out of each 64.
> The amount of memory traffic is not going to be greater (might be
> smaller, actually) and the loop in copy_to_user() is optimized for
> bulk copy.
BTW, considering the access pattern, I would expect it to be
considerably cheaper in a lot of cases; basically, we have a copy
of userland array of 64bit values, then we do a non-trivial amount
of work and modify 16 bits out of each 64. Then we want that
propagated back to the original array. I suspect that copying just
those 16bit fields out is going to cost more that a bulk copy of
the entire thing, and not just on s390 and similar oddball cases.
Comments?
next prev parent reply other threads:[~2021-01-07 18:57 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-01-07 13:47 [x86] d55564cfc2: will-it-scale.per_thread_ops -5.8% regression kernel test robot
2021-01-07 17:43 ` Linus Torvalds
2021-01-07 18:33 ` Al Viro
2021-01-07 18:40 ` Al Viro
2021-01-07 18:55 ` Al Viro [this message]
2021-01-07 18:47 ` Linus Torvalds
2021-01-07 18:58 ` Al Viro
2021-01-07 19:04 ` Al Viro
2021-01-07 19:33 ` Linus Torvalds
2021-01-07 19:57 ` Al Viro
2021-01-08 9:29 ` David Laight
2021-01-08 9:37 ` David Laight
2021-01-08 9:51 ` Peter Zijlstra
2021-01-08 10:44 ` David Laight
2021-01-08 9:49 ` David Laight
2021-01-08 6:13 ` Oliver Sang
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