perf: massive perf_event slowdown between 4.9 and 4.11-rc

perf: massive perf_event slowdown between 4.9 and 4.11-rc

[Vince Weaver]

Hello

My student actually noticed this before I did, I was hoping it was some 
sort of error in her data.

Anyway all perf_event functionality (especially reads) has become about 
20x slower, at least on Intel machines (haswell and skylake are the only 
ones I've tested) sometime between 4.9 and 4.11-rc

For example, in the PAPI tests:

4.11-rc2

Total cost for PAPI_read (2 counters) over 1000000 iterations
min cycles   : 15192
max cycles   : 3887735
mean cycles  : 15662.057418
std deviation: 19079.398693


4.9

Total cost for PAPI_read (2 counters) over 1000000 iterations
min cycles   : 864
max cycles   : 78459
mean cycles  : 908.010315
std deviation: 144.875697


The perf_event_test validation tests are also showing this, even when 
using rdpmc() rather than read.

Is there a likely change that might have caused this?  Hoping to avoid 
bisecting it as that will kill the rest of the week probably.

Vince

Re: perf: massive perf_event slowdown between 4.9 and 4.11-rc

[Vince Weaver]

On Thu, 16 Mar 2017, Vince Weaver wrote:

> Anyway all perf_event functionality (especially reads) has become about 
> 20x slower, at least on Intel machines (haswell and skylake are the only 
> ones I've tested) sometime between 4.9 and 4.11-rc

False alarm, I forgot I had debugging (KASAN, etc) turned on in this 
kernel tree because I had been fuzzing.

sorry about that.

Vince

Re: perf: massive perf_event slowdown between 4.9 and 4.11-rc

[Peter Zijlstra]

On Thu, Mar 16, 2017 at 01:57:31PM -0400, Vince Weaver wrote:
> On Thu, 16 Mar 2017, Vince Weaver wrote:
> 
> > Anyway all perf_event functionality (especially reads) has become about 
> > 20x slower, at least on Intel machines (haswell and skylake are the only 
> > ones I've tested) sometime between 4.9 and 4.11-rc
> 
> False alarm, I forgot I had debugging (KASAN, etc) turned on in this 
> kernel tree because I had been fuzzing.
> 
> sorry about that.

Ha! no problem.

Re: perf: massive perf_event slowdown between 4.9 and 4.11-rc

[Peter Zijlstra]

On Thu, Mar 16, 2017 at 11:54:58AM -0400, Vince Weaver wrote:
> Hello
> 
> My student actually noticed this before I did, I was hoping it was some 
> sort of error in her data.
> 
> Anyway all perf_event functionality (especially reads) has become about 
> 20x slower, at least on Intel machines (haswell and skylake are the only 
> ones I've tested) sometime between 4.9 and 4.11-rc
> 
> For example, in the PAPI tests:
> 
> 4.11-rc2
> 
> Total cost for PAPI_read (2 counters) over 1000000 iterations
> min cycles   : 15192
> max cycles   : 3887735
> mean cycles  : 15662.057418
> std deviation: 19079.398693
> 
> 
> 4.9
> 
> Total cost for PAPI_read (2 counters) over 1000000 iterations
> min cycles   : 864
> max cycles   : 78459
> mean cycles  : 908.010315
> std deviation: 144.875697
> 
> 
> The perf_event_test validation tests are also showing this, even when 
> using rdpmc() rather than read.
> 
> Is there a likely change that might have caused this?  Hoping to avoid 
> bisecting it as that will kill the rest of the week probably.

No immediate clue; but I can have a poke. Do you have a handy way of
showing this using: perf test rdpmc, or should I hack something
together?