linux-kernel - Re: [PATCH v3 09/19] x86/resctrl: Queue mon_event

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Date:   Thu, 23 Mar 2023 10:09:07 +0100
From:   Peter Newman <peternewman@...gle.com>
To:     James Morse <james.morse@....com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Fenghua Yu <fenghua.yu@...el.com>,
        Reinette Chatre <reinette.chatre@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        H Peter Anvin <hpa@...or.com>,
        Babu Moger <Babu.Moger@....com>,
        shameerali.kolothum.thodi@...wei.com,
        D Scott Phillips OS <scott@...amperecomputing.com>,
        carl@...amperecomputing.com, lcherian@...vell.com,
        bobo.shaobowang@...wei.com, tan.shaopeng@...itsu.com,
        xingxin.hx@...nanolis.org, baolin.wang@...ux.alibaba.com,
        Jamie Iles <quic_jiles@...cinc.com>,
        Xin Hao <xhao@...ux.alibaba.com>
Subject: Re: [PATCH v3 09/19] x86/resctrl: Queue mon_event_read() instead of
 sending an IPI

On Wed, Mar 22, 2023 at 3:07 PM Peter Newman <peternewman@...gle.com> wrote:
> On Mon, Mar 20, 2023 at 6:27 PM James Morse <james.morse@....com> wrote:
> >
> > x86 is blessed with an abundance of monitors, one per RMID, that can be
>
> As I explained earlier, this is not the case on AMD.
>
> > read from any CPU in the domain. MPAMs monitors reside in the MMIO MSC,
> > the number implemented is up to the manufacturer. This means when there are
> > fewer monitors than needed, they need to be allocated and freed.
> >
> > Worse, the domain may be broken up into slices, and the MMIO accesses
> > for each slice may need performing from different CPUs.
> >
> > These two details mean MPAMs monitor code needs to be able to sleep, and
> > IPI another CPU in the domain to read from a resource that has been sliced.
>
> This doesn't sound very convincing. Could mon_event_read() IPI all the
> CPUs in the domain? (after waiting to allocate and install monitors
> when necessary?)

No wait, I know that isn't correct.

As you explained it, the remote CPU needs to sleep because it may need
to atomically acquire, install, and read a CSU monitor.

It still seems possible for the mon_event_read() thread to do all the
waiting (tell remote CPU to program CSU monitor, wait, tell same remote
CPU to read monitor), but that sounds like more work that I don't see a
lot of benefit to doing today.

Can you update the changelog to just say the remote CPU needs to block
when installing a CSU monitor?

Thanks!
-Peter