Re: RFC: paravirtualizing perf_clock

From: Gleb Natapov
Date: Wed Oct 30 2013 - 10:21:24 EST

On Mon, Oct 28, 2013 at 08:58:08PM -0600, David Ahern wrote:
> On 10/28/13 7:15 AM, Peter Zijlstra wrote:
> >>Any suggestions on how to do this and without impacting performance. I
> >>noticed the MSR path seems to take about twice as long as the current
> >>implementation (which I believe results in rdtsc in the VM for x86 with
> >>stable TSC).
> >
> >So assuming all the TSCs are in fact stable; you could implement this by
> >syncing up the guest TSC to the host TSC on guest boot. I don't think
> >anything _should_ rely on the absolute TSC value.
> >
> >Of course you then also need to make sure the host and guest tsc
> >multipliers (cyc2ns) are identical, you can play games with
> >cyc2ns_offset if you're brave.
> >
> This and the method Gleb mentioned both are going to be complex and
> fragile -- based assumptions on how the perf_clock timestamps are
> generated. For example, 489223e assumes you have the tracepoint
> enabled at VM start with some means of capturing the data (e.g., a
> perf-session active).
We can think of other ways to provide tsc offset to perf.

> In both cases the end result requires piecing
> together and re-generating the VM's timestamp on the events. For
> perf this means either modifying the tool to take parameters and an
> algorithm on how to modify the timestamp or a homegrown tool to
> regenerate the file with updated timestamps.
> To back out a bit, my end goal is to be able to create and merge
> perf-events from any context on a KVM-based host -- guest userspace,
> guest kernel space, host userspace and host kernel space (userspace
> events with a perf-clock timestamp is another topic ;-)). Having the
> events generated with the proper timestamp is the simpler approach
> than trying to collect various tidbits of data, massage timestamps
> (and hoping the clock source hasn't changed) and then merge events.
So can you explain a little bit more about how this will work? You run
perf on a host and get both host and guest events? How do you pass
events from guest to host in this case?

> And then for the cherry on top a design that works across
> architectures (e.g., x86 now, but arm later).
MSR is x86 thing.

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