Re: svn commit: r237433 - in head/sys: amd64/include arm/include

On 06/22/12 10:06, Konstantin Belousov wrote:
> Author: kib
> Date: Fri Jun 22 07:06:40 2012
> New Revision: 237433
> URL: http://svn.freebsd.org/changeset/base/237433
>
> Log:
>    Implement mechanism to export some kernel timekeeping data to
>    usermode, using shared page.  The structures and functions have vdso
>    prefix, to indicate the intended location of the code in some future.
>
>    The versioned per-algorithm data is exported in the format of struct
>    vdso_timehands, which mostly repeats the content of in-kernel struct
>    timehands. Usermode reading of the structure can be lockless.
>    Compatibility export for 32bit processes on 64bit host is also
>    provided. Kernel also provides usermode with indication about
>    currently used timecounter, so that libc can fall back to syscall if
>    configured timecounter is unknown to usermode code.
>
>    The shared data updates are initiated both from the tc_windup(), where
>    a fast task is queued to do the update, and from sysctl handlers which
>    change timecounter. A manual override switch
>    kern.timecounter.fast_gettime allows to turn off the mechanism.
>
>    Only x86 architectures export the real algorithm data, and there, only
>    for tsc timecounter. HPET counters page could be exported as well, but
>    I prefer to not further glue the kernel and libc ABI there until
>    proper vdso-based solution is developed.
>
>    Minimal stubs neccessary for non-x86 architectures to still compile
>    are provided.
>
>    Discussed with:	bde
>    Reviewed by:	jhb
>    Tested by:	flo
>    MFC after:	1 month

> @@ -1360,6 +1367,7 @@ tc_windup(void)
>   #endif
>
>   	timehands = th;
> +	taskqueue_enqueue_fast(taskqueue_fast,&tc_windup_push_vdso_task);
>   }
>
>   /* Report or change the active timecounter hardware. */

This taskqueue_enqueue_fast() will schedule extra thread to run each 
time hardclock() fires. One thread may be not a big problem, but 
together with callout swi and possible other threads woken up there it 
will wake up several other CPU cores from sleep just to put them back in 
few microseconds. Now davide@ and me are trying to fix that by avoiding 
callout SWI use for simple tasks. Please, let's not create another 
problem same time.

-- 
Alexander Motin
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