Re: [PATCH] Perf: Correct Assumptions about Sample Timestamps inPasses

[David Ahern]

On 11/14/13, 3:05 AM, Ingo Molnar wrote:

> What am I missing?

I have spent quite a bit of time on this problem on this well. I think 
the flush time needs to be based on the start time of each round, not 
the minimum time observed across mmaps. I have tried the minimum time 
stamp route and it still fails often enough to be annoying.

See builtin-kvm.c, perf_kvm__mmap_read(). The problem is that it takes 
time to move from mmap to mmap and sample can come in behind you - an 
mmap that has already be scanned with a timestamp less than what is 
determined to be the lowest minimum for the samples actually read. 'perf 
kvm stat live' in a nested environment is a stress test for the problem.

I believe reading perf_clock at the start of each round and using that 
as the flush time will fix the problem (to the degree that perf_clock is 
monotonic across all cpus and sockets).

But, right now we have no means of reading the perf_clock timestamp in 
userspace.

David
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