Messages in this thread | | | Date | Tue, 26 Jan 2010 11:47:15 +0530 | From | Balbir Singh <> | Subject | Re: [PATCH] sched: cpuacct: Use bigger percpu counter batch values for stats counters |
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On Tuesday 26 January 2010 04:44 AM, Andrew Morton wrote: > On Mon, 18 Jan 2010 15:41:42 +1100 > Anton Blanchard <anton@samba.org> wrote: > >> When CONFIG_VIRT_CPU_ACCOUNTING and CONFIG_CGROUP_CPUACCT are 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 and we end up with a global spinlock in >> the scheduler. > > When one looks at the end result: > > : static void cpuacct_update_stats(struct task_struct *tsk, > : enum cpuacct_stat_index idx, cputime_t val) > : { > : struct cpuacct *ca; > : int batch; > : > : if (unlikely(!cpuacct_subsys.active)) > : return; > : > : rcu_read_lock(); > : ca = task_ca(tsk); > : > : batch = min_t(long, percpu_counter_batch * cputime_one_jiffy, INT_MAX); > : do { > : __percpu_counter_add(&ca->cpustat[idx], val, batch); > : ca = ca->parent; > : } while (ca); > : rcu_read_unlock(); > : } > > the code (which used to be quite obvious) becomes pretty unobvious. In > fact it looks quite wrong. > > Shouldn't there be a comment there explaining wtf is going on?
Andrew,
I guess a lot of the changelog and comments are in the email history, but your point on the comment is valid. Why does it look quite wrong to you?
cputime_one_jiffy tells us how many cputime_t's we've gotten in one jiffy. If virtual accounting is enabled, this number is quite large, and 1 if virtual accounting is not enabled. Overall the value is set to 32 for non-virtual accounting enabled systems. On systems that support virtual accounting, the value is set to 32*cputime_per_jifffy, so the per cpu counter syncs up roughly once in 32 jiffies assuming cpuacct_update_stats is called once per jiffy for non-virtual machines.
If the above comment, pleases you I'll polish it and send it across. Anton, could you please confirm what I've said above is indeed correct.
-- Three Cheers, Balbir Singh
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