Re: [RFC 0/5] CR4 handling improvements

[Peter Zijlstra]

On Tue, Oct 21, 2014 at 11:00:26AM -0400, Vince Weaver wrote:
> On Mon, 20 Oct 2014, Andy Lutomirski wrote:
> 
> > ISTM it would be a lot better to use the perf subsystem for this.  You
> > can probably pin an event to a pmu.  
> 
> No, you cannot pin an event to a counter with perf_event.
> That's one of the big differences between perf_event and, say, perfmon2.
> 
> With perf_event the kernel controls which events go in which counters and 
> the user has no say.  That's part of why you need to check the mmap page 
> every time you want to use rdpmc because there's no other way of knowing 
> which counter to read to get the event you want.
> 
> perf_event is also fairly high overhead for setting up and starting 
> events,

Which you only do once at the start, so is that really a problem?

> and mildly high overhead when doing a proper rdpmc call (due to 
> the required looking at mmap, and the fact that you need to do two rdpmc 
> calls before/after to get your value).  This is why people really worried 
> about low-latency measurements bypass as much of perf_event as possible.

I still don't get that argument, 2 rdpmc's is cheaper than doing wrmsr,
not to mention doing wrmsr through a syscall. And looking at that mmap
page is 1 cacheline. Is that cacheline read (assuming you miss) the real
problem?
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