Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > linux.kernel > #1699035

Re: [Question]: try to fix contention between expire_timers and try_to_del_timer_sync

Show all headers | View raw

On 2017-07-28 02:28, Peter Zijlstra wrote:
> On Thu, Jul 27, 2017 at 06:10:34PM -0700, Vikram Mulukutla wrote:
> 
>> I think we should have this discussion now - I brought this up earlier 
>> [1]
>> and I promised a test case that I completely forgot about - but here 
>> it
>> is (attached). Essentially a Big CPU in an acquire-check-release loop
>> will have an unfair advantage over a little CPU concurrently 
>> attempting
>> to acquire the same lock, in spite of the ticket implementation. If 
>> the Big
>> CPU needs the little CPU to make forward progress : livelock.
> 
> This needs to be fixed in hardware. There really isn't anything the
> software can sanely do about it.
> 
> It also doesn't have anything to do with the spinlock implementation.
> Ticket or not, its a fundamental problem of LL/SC. Any situation where
> we use atomics for fwd progress guarantees this can happen.
> 

Agreed, it seems like trying to build a fair SW protocol over unfair HW.
But if we can minimally change such loop constructs to address this (all
instances I've seen so far use cpu_relax) it would save a lot of hours
spent debugging these problems. Lot of b.L devices out there :-)

It's also possible that such a workaround may help contention 
performance
since the big CPU may have to wait for say a tick before breaking out of
that loop (the non-livelock scenario where the entire loop isn't in a
critical section).

> The little core (or really any core) should hold on to the locked
> cacheline for a while and not insta relinquish it. Giving it a chance 
> to
> reach the SC.

Thanks,
Vikram

-- 
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
a Linux Foundation Collaborative Project

Back to linux.kernel | Previous | Next — Previous in thread | Find similar | Unroll thread

Thread

Re: [Question]: try to fix contention between expire_timers and  try_to_del_timer_sync Thomas Gleixner <tglx@linutronix.de> - 2017-07-26 16:20 +0200
  Re: [Question]: try to fix contention between expire_timers and  try_to_del_timer_sync qiaozhou <qiaozhou@asrmicro.com> - 2017-07-27 03:40 +0200
    Re: [Question]: try to fix contention between expire_timers and  try_to_del_timer_sync Thomas Gleixner <tglx@linutronix.de> - 2017-07-27 17:20 +0200
    Re: [Question]: try to fix contention between expire_timers and  try_to_del_timer_sync Will Deacon <will.deacon@arm.com> - 2017-07-27 17:20 +0200
    Re: [Question]: try to fix contention between expire_timers and  try_to_del_timer_sync Vikram Mulukutla <markivx@codeaurora.org> - 2017-07-28 03:20 +0200
      Re: [Question]: try to fix contention between expire_timers and  try_to_del_timer_sync Will Deacon <will.deacon@arm.com> - 2017-07-28 11:30 +0200
        Re: [Question]: try to fix contention between expire_timers and  try_to_del_timer_sync Vikram Mulukutla <markivx@codeaurora.org> - 2017-07-28 21:10 +0200
          Re: [Question]: try to fix contention between expire_timers and  try_to_del_timer_sync qiaozhou <qiaozhou@asrmicro.com> - 2017-07-31 13:30 +0200
      Re: [Question]: try to fix contention between expire_timers and  try_to_del_timer_sync Peter Zijlstra <peterz@infradead.org> - 2017-07-28 11:30 +0200
        Re: [Question]: try to fix contention between expire_timers and  try_to_del_timer_sync Vikram Mulukutla <markivx@codeaurora.org> - 2017-07-28 21:20 +0200

csiph-web