Messages in this thread | | | Date | Tue, 17 Jun 2014 17:00:14 -0700 | From | Josh Triplett <> | Subject | Re: [bisected] pre-3.16 regression on open() scalability |
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On Tue, Jun 17, 2014 at 04:10:29PM -0700, Dave Hansen wrote: > On 06/13/2014 03:45 PM, Paul E. McKenney wrote: > >> > Could the additional RCU quiescent states be causing us to be doing more > >> > RCU frees that we were before, and getting less benefit from the lock > >> > batching that RCU normally provides? > > Quite possibly. One way to check would be to use the debugfs files > > rcu/*/rcugp, which give a count of grace periods since boot for each > > RCU flavor. Here "*" is rcu_preempt for CONFIG_PREEMPT and rcu_sched > > for !CONFIG_PREEMPT. > > With the previously-mentioned workload, rcugp's "age" averages 9 with > the old kernel (or RCU_COND_RESCHED_LIM at a high value) and 2 with the > current kernel which contains this regression. > > I also checked the rate and sources for how I'm calling cond_resched. > I'm calling it 5x for every open/close() pair in my test case, which > take about 7us. So, _cond_resched() is, on average, only being called > every microsecond. That doesn't seem _too_ horribly extreme. > > > 3895.165846 | 8) | SyS_open() { > > 3895.165846 | 8) 0.065 us | _cond_resched(); > > 3895.165847 | 8) 0.064 us | _cond_resched(); > > 3895.165849 | 8) 2.406 us | } > > 3895.165849 | 8) 0.199 us | SyS_close(); > > 3895.165850 | 8) | do_notify_resume() { > > 3895.165850 | 8) 0.063 us | _cond_resched(); > > 3895.165851 | 8) 0.069 us | _cond_resched(); > > 3895.165852 | 8) 0.060 us | _cond_resched(); > > 3895.165852 | 8) 2.194 us | } > > 3895.165853 | 8) | SyS_open() { > > The more I think about it, the more I think we can improve on a purely > call-based counter. > > First, it couples the number of cond_resched() directly calls with the > benefits we see out of RCU. We really don't *need* to see more grace > periods if we have more cond_resched() calls. > > It also ends up eating a new cacheline in a bunch of pretty hot paths. > It would be nice to be able to keep the fast path part of this as at > least read-only. > > Could we do something (functionally) like the attached patch? Instead > of counting cond_resched() calls, we could just specify some future time > by which we want have a quiescent state. We could even push the time to > be something _just_ before we would have declared a stall.
Looks quite promising to me, as long as the CPU in question is actively updating jiffies. I'd love to see some numbers from that approach.
- Josh Triplett
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