Date:   Tue, 14 Dec 2021 09:25:58 -0300
From:   Marcelo Tosatti <mtosatti@redhat.com>
To:     Christoph Lameter <cl@gentwo.org>
Cc:     Mel Gorman <mgorman@suse.de>, Thomas Gleixner <tglx@linutronix.de>,
        Frederic Weisbecker <frederic@kernel.org>,
        Christoph Lameter <cl@gentwo.de>,
        Nicolas Saenz Julienne <nsaenzju@redhat.com>,
        akpm@linux-foundation.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, peterz@infradead.org, nilal@redhat.com,
        linux-rt-users@vger.kernel.org, vbabka@suse.cz, ppandit@redhat.com
Subject: Re: [PATCH v2 3/3] mm/page_alloc: Remotely drain per-cpu lists
Message-ID: <20211214122558.GB57271@fuller.cnet>
References: <20211103170512.2745765-1-nsaenzju@redhat.com>
 <20211103170512.2745765-4-nsaenzju@redhat.com>
 <20211203141306.GG3301@suse.de>
 <20211209174535.GA70283@fuller.cnet>
 <20211210105549.GJ3301@suse.de>
 <20211214105823.GA53878@fuller.cnet>
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On Tue, Dec 14, 2021 at 12:42:58PM +0100, Christoph Lameter wrote:
> On Tue, 14 Dec 2021, Marcelo Tosatti wrote:
> 
> > > downside is that there will be a performance penalty if an application
> > > running on a NOHZ_FULL CPU is page allocator intensive for whatever
> > > reason.  However, I guess this is unlikely because if there was a lot
> > > of kernel activity for a NOHZ_FULL CPU, the vmstat shepherd would also
> > > cause interference.
> >
> > Yes, it does, and its being fixed:
> >
> > https://lkml.org/lkml/2021/12/8/663
> >
> > Honestly i am not sure whether the association between a nohz_full CPU
> > and "should be mostly in userspace" is desired. The RCU solution
> > would be more generic. As Nicolas mentioned, for the usecases in
> > questions, either solution is OK.
> >
> > Thomas, Frederic, Christoph, do you have any opinion on this ?
> 
> Applications running would ideally have no performance penalty and there
> is no  issue with kernel activity unless the application is in its special
> low latency loop. NOHZ is currently only activated after spinning in that
> loop for 2 seconds or so. Would be best to be able to trigger that
> manually somehow.

Can add a task isolation feature to do that.

> And I would prefer to be able to run the whole system as
> NOHZ and have the ability to selectively enable the quiet mode if a
> process requires it for its processing.

IIRC Frederic has been working on that.

Thanks.