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From: David Laight <David.Laight@ACULAB.COM>
To: 'Al Viro' <viro@zeniv.linux.org.uk>,
	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" <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" <zhengjun.xing@intel.com>
Subject: RE: [x86] d55564cfc2: will-it-scale.per_thread_ops -5.8% regression
Date: Fri, 8 Jan 2021 09:49:20 +0000	[thread overview]
Message-ID: <cc0646c570f4489bb1bfb20688d64dbe@AcuMS.aculab.com> (raw)
In-Reply-To: <20210107183358.GG3579531@ZenIV.linux.org.uk>

From: Al Viro
> Sent: 07 January 2021 18:34
> 
> On Thu, Jan 07, 2021 at 09:43:54AM -0800, Linus Torvalds wrote:
> 
> > Before, it would do the whole CLAC/STAC dance inside that loop for
> > every entry (and with that commit d55564cfc22 it would be a function
> > call, of course).
> >
> > Can you verify that this fixes the regression (and in fact I'd expect
> > it to improve that test-case)?
> 
> I'm not sure it's the best approach, TBH.  How about simply
>         for (walk = head; walk; ufds += walk->len, walk = walk->next) {
> 		if (copy_to_user(ufds, walk->entries,
> 				 walk->len * sizeof(struct pollfd))
> 			goto out_fds;
>         }
> in there?  It's both simpler (obviously matches the copyin side) and
> might very well be faster...

I bet it isn't, especially if 'user copy hardening' is enabled.
You also have to 'negotiate' all the conditionals in copy_to_user()
for misaligned lengths, very short copies and alternate algorithms for
long copies.

I'll bet there is a measurable gain from having copy_to/from_user_u64()
that is optimised for copying a structure to/from userspace in 64bit
chunks.

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)


WARNING: multiple messages have this Message-ID (diff)
From: David Laight <David.Laight@ACULAB.COM>
To: lkp@lists.01.org
Subject: Re: [x86] d55564cfc2: will-it-scale.per_thread_ops -5.8% regression
Date: Fri, 08 Jan 2021 09:49:20 +0000	[thread overview]
Message-ID: <cc0646c570f4489bb1bfb20688d64dbe@AcuMS.aculab.com> (raw)
In-Reply-To: <20210107183358.GG3579531@ZenIV.linux.org.uk>

[-- Attachment #1: Type: text/plain, Size: 1279 bytes --]

From: Al Viro
> Sent: 07 January 2021 18:34
> 
> On Thu, Jan 07, 2021 at 09:43:54AM -0800, Linus Torvalds wrote:
> 
> > Before, it would do the whole CLAC/STAC dance inside that loop for
> > every entry (and with that commit d55564cfc22 it would be a function
> > call, of course).
> >
> > Can you verify that this fixes the regression (and in fact I'd expect
> > it to improve that test-case)?
> 
> I'm not sure it's the best approach, TBH.  How about simply
>         for (walk = head; walk; ufds += walk->len, walk = walk->next) {
> 		if (copy_to_user(ufds, walk->entries,
> 				 walk->len * sizeof(struct pollfd))
> 			goto out_fds;
>         }
> in there?  It's both simpler (obviously matches the copyin side) and
> might very well be faster...

I bet it isn't, especially if 'user copy hardening' is enabled.
You also have to 'negotiate' all the conditionals in copy_to_user()
for misaligned lengths, very short copies and alternate algorithms for
long copies.

I'll bet there is a measurable gain from having copy_to/from_user_u64()
that is optimised for copying a structure to/from userspace in 64bit
chunks.

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)

  parent reply	other threads:[~2021-01-08  9:51 UTC|newest]

Thread overview: 32+ 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 13:47 ` kernel test robot
2021-01-07 17:43 ` Linus Torvalds
2021-01-07 17:43   ` Linus Torvalds
2021-01-07 18:33   ` Al Viro
2021-01-07 18:33     ` Al Viro
2021-01-07 18:40     ` Al Viro
2021-01-07 18:40       ` Al Viro
2021-01-07 18:55       ` Al Viro
2021-01-07 18:55         ` Al Viro
2021-01-07 18:47     ` Linus Torvalds
2021-01-07 18:47       ` Linus Torvalds
2021-01-07 18:58       ` Al Viro
2021-01-07 18:58         ` Al Viro
2021-01-07 19:04       ` Al Viro
2021-01-07 19:04         ` Al Viro
2021-01-07 19:33         ` Linus Torvalds
2021-01-07 19:33           ` Linus Torvalds
2021-01-07 19:57           ` Al Viro
2021-01-07 19:57             ` Al Viro
2021-01-08  9:29             ` David Laight
2021-01-08  9:29               ` David Laight
2021-01-08  9:37           ` David Laight
2021-01-08  9:37             ` David Laight
2021-01-08  9:51             ` Peter Zijlstra
2021-01-08  9:51               ` Peter Zijlstra
2021-01-08 10:44               ` David Laight
2021-01-08 10:44                 ` David Laight
2021-01-08  9:49     ` David Laight [this message]
2021-01-08  9:49       ` David Laight
2021-01-08  6:13   ` Oliver Sang
2021-01-08  6:13     ` Oliver Sang

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