Messages in this thread | | | Date | Thu, 20 Aug 2009 10:56:05 +0530 | From | Balbir Singh <> | Subject | Re: [tip:sched/core] sched: cpuacct: Use bigger percpu counter batch values for stats counters |
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* Anton Blanchard <anton@samba.org> [2009-08-20 15:10:38]:
> > Hi, > > Looks like this issue is still present. I tested on a 32 core box and > the patch improved maximum context switch rate from from 76k/sec to 9.5M/sec. > Thats over 100x faster, or 50x per line of code. That's got to be some sort of > record :) > > Any chance we can get a fix in for 2.6.31? Don't make me find an even bigger > box so I can break the 200x mark :) > > Anton > > > -- > > > > When CONFIG_VIRT_CPU_ACCOUNTING is enabled we can call cpuacct_update_stats > > with values much larger than percpu_counter_batch. This means the > > call to percpu_counter_add will always add to the global count which is > > protected by a spinlock. > > > > Since reading of the CPU accounting cgroup counters is not performance > > critical, we can use a maximum size batch of INT_MAX and use > > percpu_counter_sum on the read side which will add all the percpu > > counters. > > > > With this patch an 8 core POWER6 with CONFIG_VIRT_CPU_ACCOUNTING and > > CONFIG_CGROUP_CPUACCT shows an improvement in aggregate context switch rate of > > 397k/sec to 3.9M/sec, a 10x improvement. > >
Looks good overall, why not keep the batch size conditional on CONFIG_VIRT_CPU_ACCOUNTING? I'd still like to stick with percpu_counter_read() on the read side because My concern is that a bad user space application can read cpuacct.stat and bring the kernel to its knees.
> > Signed-off-by: Anton Blanchard <anton@samba.org> > > --- > > > > Index: linux.trees.git/kernel/sched.c > > =================================================================== > > --- linux.trees.git.orig/kernel/sched.c 2009-07-16 10:11:02.000000000 +1000 > > +++ linux.trees.git/kernel/sched.c 2009-07-16 10:16:41.000000000 +1000 > > @@ -10551,7 +10551,7 @@ > > int i; > > > > for (i = 0; i < CPUACCT_STAT_NSTATS; i++) { > > - s64 val = percpu_counter_read(&ca->cpustat[i]); > > + s64 val = percpu_counter_sum(&ca->cpustat[i]); > > val = cputime64_to_clock_t(val); > > cb->fill(cb, cpuacct_stat_desc[i], val); > > } > > @@ -10621,7 +10621,7 @@ > > ca = task_ca(tsk); > > > > do { > > - percpu_counter_add(&ca->cpustat[idx], val); > > + __percpu_counter_add(&ca->cpustat[idx], val, INT_MAX); > > ca = ca->parent; > > } while (ca); > > rcu_read_unlock();
-- Balbir
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