linux-kernel - Re: [patch 1/4] add basic task isolation prctl interface

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Date:   Wed, 28 Jul 2021 13:55:33 +0200
From:   nsaenzju@...hat.com
To:     Marcelo Tosatti <mtosatti@...hat.com>,
        Frederic Weisbecker <frederic@...nel.org>
Cc:     linux-kernel@...r.kernel.org, Nitesh Lal <nilal@...hat.com>,
        Christoph Lameter <cl@...ux.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Alex Belits <abelits@...vell.com>,
        Peter Xu <peterx@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [patch 1/4] add basic task isolation prctl interface

Hi Marcelo,

On Wed, 2021-07-28 at 06:37 -0300, Marcelo Tosatti wrote:
> On Wed, Jul 28, 2021 at 01:45:39AM +0200, Frederic Weisbecker wrote:
> > On Tue, Jul 27, 2021 at 11:52:09AM -0300, Marcelo Tosatti wrote:
> > > The meaning of isolated is specified as follows:
> > > 
> > > Isolation features
> > > ==================
> > > 
> > > - prctl(PR_ISOL_GET, ISOL_SUP_FEATURES, 0, 0, 0) returns the supported
> > > features as a return value.
> > > 
> > > - prctl(PR_ISOL_SET, ISOL_FEATURES, bitmask, 0, 0) enables the features in
> > > the bitmask.
> > > 
> > > - prctl(PR_ISOL_GET, ISOL_FEATURES, 0, 0, 0) returns the currently
> > > enabled features.
> > 
> > So what are the ISOL_FEATURES here? A mode that we enter such as flush
> > vmstat _everytime_ we resume to userpace after (and including) this prctl() ?
> 
> ISOL_FEATURES is just the "command" type (which you can get and set).
> 
> The bitmask would include ISOL_F_QUIESCE_ON_URET, so:
> 
> - bitmask = ISOL_F_QUIESCE_ON_URET;
> - prctl(PR_ISOL_SET, ISOL_FEATURES, bitmask, 0, 0) enables the features in
> the bitmask.
> 
> - quiesce_bitmap = prctl(PR_ISOL_GET, PR_ISOL_SUP_QUIESCE_CFG, 0, 0, 0)
>   (1)
> 
>   (returns the supported actions to be quiesced).
> 
> - prctl(PR_ISOL_SET, PR_ISOL_QUIESCE_CFG, quiesce_bitmask, 0, 0) _sets_
> the actions to be quiesced (2)
> 
> If an application does not modify "quiesce_bitmask" between 
> points (1) and (2) above, it will enable quiescing of all
> "features" the kernel supports.

I think this pattern of enabling all by default might be prone to subtly
breaking things.

For example, let's say we introduce ISOL_F_QUIESCE_DEFER_TLB_FLUSH, this will
defer relatively short IPIs on isolated CPUs in exchange for a longer flush
whenever we enter the kernel (syscall, IRQs, NMI, etc...). A latency sensitive
application might be OK with the former but not with the latter.

-- 
Nicolás Sáenz