Re: [PATCH] perf/core: generate overflow signal when samples are dropped (WAS: Re: [REGRESSION] perf/core: PMU interrupts dropped if we entered the kernel in the "skid" region)

From: Kyle Huey
Date: Tue Jul 11 2017 - 11:32:50 EST

On Tue, Jul 11, 2017 at 2:03 AM, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
> * Kyle Huey <me@xxxxxxxxxxxx> wrote:
>> On Wed, Jul 5, 2017 at 10:07 PM, Robert O'Callahan <robert@xxxxxxxxxxxxx> wrote:
>> > On Tue, Jul 4, 2017 at 3:21 AM, Mark Rutland <mark.rutland@xxxxxxx> wrote:
>> >> Should any of those be moved into the "should be dropped" pile?
>> >
>> > Why not be conservative and clear every sample you're not sure about?
>> >
>> > We'd appreciate a fix sooner rather than later here, since rr is
>> > currently broken on every stable Linux kernel and our attempts to
>> > implement a workaround have failed.
>> >
>> > (We have separate "interrupt" and "measure" counters, and I thought we
>> > might work around this regression by programming the "interrupt"
>> > counter to count kernel events as well as user events (interrupting
>> > early is OK), but that caused our (completely separate) "measure"
>> > counter to report off-by-one results (!), which seems to be a
>> > different bug present on a range of older kernels.)
>> This seems to have stalled out here unfortunately.
>> Can we get a consensus (from ingo or peterz?) on Mark's question? Or,
>> alternatively, can we move the patch at the top of this thread forward
>> on the stable branches until we do reach an answer to that question?
>> We've abandoned hope of working around this problem in rr and are
>> currently broken for all of our users with an up-to-date kernel, so
>> the situation for us is rather dire at the moment I'm afraid.
> Sorry about that - I've queued up a revert for the original commit and will send
> the fix to Linus later today. I've added a -stable tag as well so it can be
> forwarded to Greg the moment it hits upstream.

Great, thank you.

- Kyle