linux-kernel - Re: sched/debug: CPU hotplug operation suffers in a large cpu systems

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Message-ID: <Y5iQJqCoxJH+GcqH@lorien.usersys.redhat.com>
Date:   Tue, 13 Dec 2022 09:45:58 -0500
From:   Phil Auld <pauld@...hat.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Vishal Chourasia <vishalc@...ux.vnet.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org, mingo@...hat.com,
        vincent.guittot@...aro.org, vschneid@...hat.com,
        srikar@...ux.vnet.ibm.com, sshegde@...ux.ibm.com,
        linuxppc-dev@...ts.ozlabs.org, ritesh.list@...il.com,
        aneesh.kumar@...ux.ibm.com
Subject: Re: sched/debug: CPU hotplug operation suffers in a large cpu systems

On Tue, Dec 13, 2022 at 03:31:06PM +0100 Greg Kroah-Hartman wrote:
> On Tue, Dec 13, 2022 at 08:22:58AM -0500, Phil Auld wrote:
> > On Tue, Dec 13, 2022 at 07:23:54AM +0100 Greg Kroah-Hartman wrote:
> > > On Mon, Dec 12, 2022 at 02:17:58PM -0500, Phil Auld wrote:
> > > > Hi,
> > > > 
> > > > On Tue, Nov 08, 2022 at 01:24:39PM +0100 Greg Kroah-Hartman wrote:
> > > > > On Tue, Nov 08, 2022 at 03:30:46PM +0530, Vishal Chourasia wrote:
> > > > > > 
> > > > > > Thanks Greg & Peter for your direction. 
> > > > > > 
> > > > > > While we pursue the idea of having debugfs based on kernfs, we thought about
> > > > > > having a boot time parameter which would disable creating and updating of the
> > > > > > sched_domain debugfs files and this would also be useful even when the kernfs
> > > > > > solution kicks in, as users who may not care about these debugfs files would
> > > > > > benefit from a faster CPU hotplug operation.
> > > > > 
> > > > > Ick, no, you would be adding a new user/kernel api that you will be
> > > > > required to support for the next 20+ years.  Just to get over a
> > > > > short-term issue before you solve the problem properly.
> > > > 
> > > > I'm not convinced moving these files from debugfs to kernfs is the right
> > > > fix.  That will take it from ~50 back to ~20 _minutes_ on these systems.
> > > > I don't think either of those numbers is reasonable.
> > > > 
> > > > The issue as I see it is the full rebuild for every change with no way to
> > > > batch the changes. How about something like the below?
> > > > 
> > > > This puts the domains/* files under the sched_verbose flag. About the only
> > > > thing under that flag now are the detailed topology discovery printks anyway
> > > > so this fits together nicely.
> > > > 
> > > > This way the files would be off by default (assuming you don't boot with
> > > > sched_verbose) and can be created at runtime by enabling verbose. Multiple
> > > > changes could also be batched by disabling/makeing changes/re-enabling.
> > > > 
> > > > It does not create a new API, uses one that is already there.
> > > 
> > > The idea seems good, the implementation might need a bit of work :)
> > 
> > More than the one comment below? Let me know.
> 
> No idea, resubmit a working patch and I'll review it properly :)
>

Will do. 


Thanks,
Phil


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