linux-kernel - Re: [x86] d55564cfc2: will-it-scale.per_thread

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Date:   Thu, 7 Jan 2021 18:33:58 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     kernel test robot <oliver.sang@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>, lkp@...ts.01.org,
        kernel test robot <lkp@...el.com>,
        "Huang, Ying" <ying.huang@...el.com>,
        Feng Tang <feng.tang@...el.com>, zhengjun.xing@...el.com
Subject: Re: [x86] d55564cfc2: will-it-scale.per_thread_ops -5.8% regression

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...