[lkml]   [2011]   [Dec]   [21]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
SubjectRe: [PATCH 0/3] coupled cpuidle state support
On Wed, Dec 21, 2011 at 1:44 AM, Arjan van de Ven <> wrote:
> On 12/21/2011 10:40 AM, Colin Cross wrote:
>>> this smells fundamentally racey to me; you can get an interrupt one
>>> cycle after you think you're done, but before the last guy enters WFI...
>>> how do you solve that issue ?
>> All the cpus have interrupts off when they increment the counter, so
>> they cannot receive an interrupt.  If an interrupt is pending on one
>> of those cpus, it will be handled later when WFI aborts due to the
>> pending interrupt.
> ... but this leads to cases where you're aborting before other cpus are
> entering..... so your "last guy in" doesn't really work, since while cpu
> 0 thinks it's the last guy, cpu 1 is already on the way out/out
> already...  (heck it might already be going back to sleep if your idle
> code can run fast, like in the size of a cache miss)

Once a cpu has incremented the counter, it has no way out unless either
1: another cpu (that hasn't incremented the counter yet) receives an
interrupt, aborts idle, and clears its idle flag
2: all cpus enter the ready counter, and call the cpuidle driver's
enter function.

In your example, cpu 1 has incremented the counter, so it cannot be on
the way out unless cpu 0 aborts (in which case it will not increment
the counter, and the counter will never be equal to the number of
cpus), or unless cpu 0 turns off its interrupts and incrementes the
counter (in which case neither cpu can return until after the cpuidle
driver's enter function has been called on all cpus).
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to
More majordomo info at
Please read the FAQ at

 \ /
  Last update: 2011-12-21 10:57    [W:0.081 / U:0.628 seconds]
©2003-2018 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site