Re: [PATCH] ia64: disable preemption in udelay()

[Keith Owens <kaos@sgi.com>]

On Thu, 22 Dec 2005 16:45:08 -0500, 
Bill Davidsen <davidsen@tmr.com> wrote:
>Lee Revell wrote:
>> On Thu, 2005-12-15 at 18:12 -0800, John Hawkes wrote:
>> 
>>>From: "Lee Revell" <rlrevell@joe-job.com>
>>>
>>>>There are 10 drivers that udelay(10000) or more and a TON that
>>>>udelay(1000).  Turning those all into 1ms+ non preemptible sections will
>>>>be very bad.
>>>
>>>What about 100usec non-preemptible sections?
>> 
>> 
>> That will disappear into the noise, in normal usage these happen all the
>> time.  500usec non preemptible regions are rare (~1 hour to show up) and
>> 1ms very rare (24 hours).  My tests show that 300 usec or so is a good
>> place to draw the line if you don't want it to show up in latency tests.
>
>I may be misreading the original post, but the problem is described as 
>one where the TSC is not syncronised and a CPU switch takes place. Would 
>the correct solution be to somehow set CPU affinity temporarily in such 
>a way as to avoid disabling preempt at all?
>
>The preempt doesn't seem to be the root problem, so it's unlikely to be 
>the best solution...

Agreed.  See [RFC] Add thread_info flag for "no cpu migration"[1] on
lkml.  It got no response.

[1] http://marc.theaimsgroup.com/?l=linux-kernel&m=113471059115107&w=2