linux-kernel - RE: [PATCH 00/10] x86/CPU and x86/resctrl: Support pseudo-lock regions spanning L2 and L3 cache

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Message-ID: <41cd71514a9042abaaef909d816e2522@AcuMS.aculab.com>
Date:   Thu, 27 Jun 2019 09:12:40 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Reinette Chatre' <reinette.chatre@...el.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "fenghua.yu@...el.com" <fenghua.yu@...el.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "tony.luck@...el.com" <tony.luck@...el.com>
CC:     "mingo@...hat.com" <mingo@...hat.com>,
        "hpa@...or.com" <hpa@...or.com>, "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH 00/10] x86/CPU and x86/resctrl: Support pseudo-lock
 regions spanning L2 and L3 cache

From: Reinette Chatre
> Sent: 26 June 2019 18:49
> 
> Cache pseudo-locking involves preloading a region of physical memory into a
> reserved portion of cache that no task or CPU can subsequently fill into and
> from that point on will only serve cache hits. At this time it is only
> possible to create cache pseudo-locked regions in either L2 or L3 cache,
> supporting systems that support either L2 Cache Allocation Technology (CAT)
> or L3 CAT because CAT is the mechanism used to manage reservations of cache
> portions.

While this is a 'nice' hardware feature for some kinds of embedded systems
I don't see how it can be sensibly used inside a Linux kernel.
There are an awful lot of places where things can go horribly wrong.
I can imagine:
- Multiple requests to lock regions that end up trying to use the same
  set-associative cache lines leaving none for normal operation.
- Excessive cache line bouncing because fewer lines are available.
- The effect of cache invalidate requests for the locked addresses.
- I suspect the Linux kernel can do full cache invalidates at certain times.

You've not given a use case.

	David

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