Messages in this thread | | | Date | Mon, 9 Feb 2009 12:48:52 +0100 | From | Ingo Molnar <> | Subject | Re: [PATCH] perf_counter: Make software counters work as per-cpu counters |
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* Paul Mackerras <paulus@samba.org> wrote:
> Impact: kernel crash fix > > Yanmin Zhang reported that using a PERF_COUNT_TASK_CLOCK software > counter as a per-cpu counter would reliably crash the system, because > it calls __task_delta_exec with a null pointer. The page fault, > context switch and cpu migration counters also won't function > correctly as per-cpu counters since they reference the current task. > > This fixes the problem by redirecting the task_clock counter to the > cpu_clock counter when used as a per-cpu counter, and by implementing > per-cpu page fault, context switch and cpu migration counters. > > Along the way, this: > > - Initializes counter->ctx earlier, in perf_counter_alloc, so that > sw_perf_counter_init can use it > - Adds code to kernel/sched.c to count task migrations into each > cpu, in rq->nr_migrations_in > - Exports the per-cpu context switch and task migration counts > via new functions added to kernel/sched.c > - Makes sure that if sw_perf_counter_init fails, we don't try to > initialize the counter as a hardware counter. Since the user has > passed a negative, non-raw event type, they clearly don't intend > for it to be interpreted as a hardware event. > > Signed-off-by: Paul Mackerras <paulus@samba.org>
Very nice, thanks Paul!
> I'm a little concerned about the use of u64 for the existing > rq->nr_switches and the new rq->nr_migrations_in. On 32-bit machines > this will get updated in two halves, so there is a very small but > non-zero probability that reading it will give a bogus value. It's > not clear to me what the best way to fix it is, since not all 32-bit > platforms have atomic64_t. Maybe make nr_switches and > nr_migrations_in unsigned long, or atomic_t?
It used to be 'just stats' so we dont really mind. But now that we expose it in a bit more systematic way i agree that it should be fixed. Updating to 'unsigned long' sounds good to me.
> I have some sort of git problem with my perfcounters.git repository. > I'll let you know when I have that sorted out.
ok - i've applied your patch from email to allow Yanmin to test tip:master.
Ingo
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