Re: [PATCH] sched: cpuacct: Use bigger percpu counter batch values for stats counters

Anton Blanchard
2010-01-26T08:11:01Z

Hi Andrew,

> > I guess a lot of the changelog and comments are in the email history,
>
> Not a very useful location for it!

Good point, I'll work on a useful comment.

> > 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.

On ppc64, s390 and ia64 cputime_one_jiffy is 64bit and I want to prevent us
creating too large a batch value:

void __percpu_counter_add(struct percpu_counter *fbc, s64 amount, s32 batch)

If we pass in something bigger than INT_MAX we could end up with 0 after
truncation which will turn the percpu counter into a single spinlock global
counter.

Anton