Messages in this thread | | | Date | Mon, 25 Jan 2010 22:35:13 -0800 | From | Andrew Morton <> | Subject | Re: [PATCH] sched: cpuacct: Use bigger percpu counter batch values for stats counters |
| |
On Tue, 26 Jan 2010 11:47:15 +0530 Balbir Singh <balbir@linux.vnet.ibm.com> wrote:
> 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,
Not a very useful location for it!
> Why does it look quite wrong to you?
Because it computes the correct value and then if it's larger than INT_MAX, it inexplicably assigns INT_MAX to it, giving a wrong result!
Does that code actually work, btw? percpu_counter_batch has type `int' and cputime_one_jiffy has type `int' so their product has type `int'. So by the time min_t performs its comparison, the upper 32 bits of the product are already lost.
| |