linux-kernel - Re: [PATCH 05/32] x86/intel_rdt: Implement scheduling support for Intel RDT

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Date:	Mon, 25 Jul 2016 15:47:54 -0700
From:	David Carrillo-Cisneros <davidcc@...gle.com>
To:	"Luck, Tony" <tony.luck@...el.com>,
	Nilay Vaish <nilayvaish@...il.com>
Cc:	Fenghua Yu <fenghua.yu@...el.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>,
	"H. Peter Anvin" <h.peter.anvin@...el.com>,
	Tejun Heo <tj@...nel.org>, Borislav Petkov <bp@...e.de>,
	Stephane Eranian <eranian@...gle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Ravi V Shankar <ravi.v.shankar@...el.com>,
	Vikas Shivappa <vikas.shivappa@...ux.intel.com>,
	Sai Prakhya <sai.praneeth.prakhya@...el.com>,
	linux-kernel <linux-kernel@...r.kernel.org>, x86 <x86@...nel.org>
Subject: Re: [PATCH 05/32] x86/intel_rdt: Implement scheduling support for
 Intel RDT

On Mon, Jul 25, 2016 at 11:05 AM Luck, Tony <tony.luck@...el.com> wrote:
>
> On Mon, Jul 25, 2016 at 11:31:24AM -0500, Nilay Vaish wrote:
> > I was thinking more about this software caching of CLOSids.  How
> > likely do you think these CLOSids would be found cached?  I think the
> > software cache would be very infrequently accessed, so it seems you
> > are likely to miss these in all levels of cache hierarchy and more
> > likely to have to fetch these from the main memory, which itself might
> > cost ~250 cycles.
>
> We need to avoid reading the PQR_ASSOC MSR (which would cost far
> more than 250 cycles).  Life is complicated here because this
> MSR contains the CLOSID in the upper half, and the RMID (owned
> by the perf code to measure cache occupancy and memory bandwidth)
> in the lower half.


On my Haswell machine, writing PQR_ASSOC_MSR takes about 380 cycles.
As Tony said, CQM/CMT writes to the same register, and it does it
twice (once to delete the old event, once to add the new one).

So, if a CQM/CMT or MBM is used with CAT, there will be 3 writes to
PQR_ASSOC_MSR per context switch and it's quite likely that the
software cache's cache line will be there for the 2 last writes.