lkml.org 
[lkml]   [2012]   [May]   [3]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [RFC] sched: make callers check lock contention for cond_resched_lock()
On 05/03/2012 09:00 PM, Takuya Yoshikawa wrote:

> On Thu, 03 May 2012 14:29:10 +0200
> Peter Zijlstra <peterz@infradead.org> wrote:
>
>> On Thu, 2012-05-03 at 21:22 +0900, Takuya Yoshikawa wrote:
>>> Although the real use case is out of this RFC patch, we are now discussing
>>> a case in which we may hold a spin_lock for long time, ms order, depending
>>> on workload; and in that case, other threads -- VCPU threads -- should be
>>> given higher priority for that problematic lock.
>>
>> Firstly, if you can hold a lock that long, it shouldn't be a spinlock,
>
> I agree with you in principle, but isn't cond_resched_lock() there for that?
>
>> secondly why isn't TIF_RESCHED being set if its running that long? That
>> should still make cond_resched_lock() break.
>
> I see.
>
> I did some tests using spin_is_contended() and need_resched() and saw
> that need_resched() was called as often as spin_is_contended(), so
> experimentally I understand your point.
>
> But as I could not see why spin_needbreak() was differently implemented
> depending on CONFIG_PREEMPT, I wanted to understand the meaning.


I think enable CONFIG_PREEMPT means allow preemption in kernel, so if
disabled, we can't reschedule a task if it is running in kernel not the
user space at a given time.

As the cond_resched_lock() was invoked in kernel, and looks like
cpu_relax() will give up cpu(I'm not sure whether this will invoke
schedule on some arch, just because that name...), so we can't do break
if CONFIG_PREEMPT disabled, because that will cause kernel preemption
while not allowed.

May be that's the reason why we need to consider CONFIG_PREEMPT in
spin_needbreak().

Regards,
Michael Wang

>
> Thanks,
> Takuya
> --
> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at http://vger.kernel.org/majordomo-info.html
> Please read the FAQ at http://www.tux.org/lkml/
>




\
 
 \ /
  Last update: 2012-05-04 05:01    [W:0.123 / U:0.120 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site