linux-kernel - Re: [PATCH] sched: Support current clocksource handling in fallback sched

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Tue, 26 May 2009 17:02:58 +0200
From:	Matthieu CASTET <matthieu.castet@...rot.com>
To:	Linus Walleij <linus.ml.walleij@...il.com>
CC:	Paul Mundt <lethal@...ux-sh.org>, Ingo Molnar <mingo@...e.hu>,
	Andrew Victor <linux@...im.org.za>,
	Haavard Skinnemoen <hskinnemoen@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-sh@...r.kernel.org" <linux-sh@...r.kernel.org>,
	"peterz@...radead.org" <peterz@...radead.org>,
	"linux-arm-kernel@...ts.arm.linux.org.uk" 
	<linux-arm-kernel@...ts.arm.linux.org.uk>
Subject: Re: [PATCH] sched: Support current clocksource handling in fallback
 	sched_clock().

Linus Walleij a écrit :
> 2009/5/26 Paul Mundt <lethal@...ux-sh.org>:
> 
>>  */
>>  unsigned long long __attribute__((weak)) sched_clock(void)
>>  {
>> +       /*
>> +        * Use the current clocksource when it becomes available later in
>> +        * the boot process, and ensure that it has a high enough rating
>> +        * to make it suitable for general use.
>> +        */
>> +       if (clock && clock->rating >= 100)
>> +               return cyc2ns(clock, clocksource_read(clock));
>> +
>> +       /* Otherwise just fall back on jiffies */
>>        return (unsigned long long)(jiffies - INITIAL_JIFFIES)
>>                                        * (NSEC_PER_SEC / HZ);
>>  }
This is buggy for fast clocksource that wrap often, because sched_clock
is not supposed to wrap.
That why custom implementation use cnt32_to_63 and a smaller shift
(around 8 -10) for conversion for timer unit to ns.

Look for example to arch/arm/mach-pxa/time.c implementation [1]

But we can image you make it generic.

Matthieu

[1]
/*
 * This is PXA's sched_clock implementation. This has a resolution
 * of at least 308 ns and a maximum value of 208 days.
 *
 * The return value is guaranteed to be monotonic in that range as
 * long as there is always less than 582 seconds between successive
 * calls to sched_clock() which should always be the case in practice.
 */
#define OSCR2NS_SCALE_FACTOR 10

static unsigned long oscr2ns_scale;

static void __init set_oscr2ns_scale(unsigned long oscr_rate)
{
    unsigned long long v = 1000000000ULL << OSCR2NS_SCALE_FACTOR;
    do_div(v, oscr_rate);
    oscr2ns_scale = v;
    /*
     * We want an even value to automatically clear the top bit
     * returned by cnt32_to_63() without an additional run time
     * instruction. So if the LSB is 1 then round it up.
     */
    if (oscr2ns_scale & 1)
        oscr2ns_scale++;
}

unsigned long long sched_clock(void)
{
    unsigned long long v = cnt32_to_63(OSCR);
    return (v * oscr2ns_scale) >> OSCR2NS_SCALE_FACTOR;
}
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/