Re: [PATCH] x86: TSC resync

Ingo Molnar
2008-09-27T16:51:33Z

* Michael Davidson <md@google.com> wrote:

> On Fri, Sep 26, 2008 at 12:02 AM, Ingo Molnar <mingo@elte.hu> wrote:
>
> > hm, this patch syncs the TSCs every 20 seconds. That is enough to
> > sync up AMD CPUs where the TSC slows down _slightly_ (at 10 ppm per
> > second or so) when it's in HLT.
>
> Actually by default it syncs them every time the CPU comes out of idle
> or, optionally, on the first user space access to the TSC via RDTSC or
> RDTSCP after the CPU has been in idle.

hm, that would be a performance problem.

coming out of idle is a fastpath: the CPU just got notified of more
work, so any delay there costs.

[ going _to_ idle is more relaxed from a performance POV - but that
doesnt help much here. ]

and the default_idle()->tsc_resync_idle_notifier()->tsc_resync()->
ref_clock_tsc_read()->ref_clock() codepath does this:

+ offset = (uint32_t)hpet_readl(HPET_COUNTER) - ref_clock_last;

i.e. we read the HPET every time we exit from idle. The HPET usually
sits in the southbridge and thus we add ~5000 cycles of overhead to
every idle-exit event.

Another thing i noticed. You set the TSC via the 0x10 MSR:

+ asm __volatile__ (
+ "xorl %%eax, %%eax\n\t"
+ "cpuid\n\t"
+ "movl $0x10, %%ecx\n\t"
+ "rdmsr\n\t"
+ "addl %%edi, %%eax\n\t"
+ "adcl %%esi, %%edx\n\t"
+ "wrmsr\n"

some CPUs (P3's?) will crop the TSC at 32 bits when wrtsc is used [even
if a larger than 0xfffffffff value is presented in ax/dx. - i.e. the TSC
will go round and round, with a rollover every few seconds.

those CPUs would have to be detected and excluded from this scheme.

Ingo