linux-kernel - Re: [PATCH 08/21] x86/intel_rdt/cqm: Add RMID(Resource monitoring ID) management

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Message-ID: <CA+8MBb+Ty2xDWeDXGN0qC_E=BY_Pi4eiwLug-mQ6GGD2-Vmw3A@mail.gmail.com>
Date:   Wed, 5 Jul 2017 10:59:50 -0700
From:   Tony Luck <tony.luck@...il.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Vikas Shivappa <vikas.shivappa@...ux.intel.com>,
        X86-ML <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        ravi.v.shankar@...el.com, vikas.shivappa@...el.com,
        "Yu, Fenghua" <fenghua.yu@...el.com>, andi.kleen@...el.com
Subject: Re: [PATCH 08/21] x86/intel_rdt/cqm: Add RMID(Resource monitoring ID) management

> In case that a RMID was never used on a particular package, the state check
> forces an IPI on all packages unconditionally. That's suboptimal at least.
>
> We know on which package a given RMID was used, so we could restrict the
> checks to exactly these packages, but I'm not sure it's worth the
> trouble. We might at least document that and explain why this is
> implemented in that way.

We only allocate RMIDs when a user makes a directory.  I don't think
we should consider options that slow down context switch in order to
keep track of which packages were used just to make mkdir(2) a bit faster
in the case where we need to check the limbo list.

We could make the check of the limbo list less costly by using a bitmask
to keep track of which packages have already found that the llc_occupancy
is below the threshold. But I'd question whether the extra complexity in the
code was really worth it.

-Tony [on vacation - responses will be slow]