linux-kernel - Re: [PATCHv4 26/28] x86/vdso: Align VDSO functions by CPU L1 cache line

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Date:   Fri, 14 Jun 2019 16:13:31 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Dmitry Safonov <dima@...sta.com>
cc:     linux-kernel@...r.kernel.org, Andrei Vagin <avagin@...il.com>,
        Adrian Reber <adrian@...as.de>,
        Andrei Vagin <avagin@...nvz.org>,
        Andy Lutomirski <luto@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Cyrill Gorcunov <gorcunov@...nvz.org>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Jann Horn <jannh@...gle.com>, Jeff Dike <jdike@...toit.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Pavel Emelyanov <xemul@...tuozzo.com>,
        Shuah Khan <shuah@...nel.org>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        containers@...ts.linux-foundation.org, criu@...nvz.org,
        linux-api@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCHv4 26/28] x86/vdso: Align VDSO functions by CPU L1 cache
 line

On Wed, 12 Jun 2019, Dmitry Safonov wrote:

> From: Andrei Vagin <avagin@...il.com>
> 
> After performance testing VDSO patches a noticeable 20% regression was
> found on gettime_perf selftest with a cold cache.
> As it turns to be, before time namespaces introduction, VDSO functions
> were quite aligned to cache lines, but adding a new code to adjust
> timens offset inside namespace created a small shift and vdso functions
> become unaligned on cache lines.
> 
> Add align to vdso functions with gcc option to fix performance drop.
> 
> Coping the resulting numbers from cover letter:
> 
> Hot CPU cache (more gettime_perf.c cycles - the better):
>         | before     | CONFIG_TIME_NS=n | host        | inside timens
> --------|------------|------------------|-------------|-------------
> cycles  | 139887013  | 139453003        | 139899785   | 128792458
> diff (%)| 100        | 99.7             | 100         | 92

Why is CONFIG_TIME_NS=n behaving worse than current mainline and
worse than 'host' mode?

> Cold cache (lesser tsc per gettime_perf_cold.c cycle - the better):
>         | before     | CONFIG_TIME_NS=n | host        | inside timens
> --------|------------|------------------|-------------|-------------
> tsc     | 6748       | 6718             | 6862        | 12682
> diff (%)| 100        | 99.6             | 101.7       | 188

Weird, now CONFIG_TIME_NS=n is better than current mainline and 'host' mode
drops.

Either I'm misreading the numbers or missing something or I'm just confused
as usual :)

Thanks,

	tglx