linux-kernel - Re: [PATCH v2 0/2] sched: blk: Handle HMP systems when completing IO

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Date: Sat, 24 Feb 2024 23:12:13 +0000
From: Qais Yousef <qyousef@...alina.io>
To: Shrikanth Hegde <sshegde@...ux.ibm.com>
Cc: linux-kernel@...r.kernel.org, linux-block@...r.kernel.org,
	Sudeep Holla <sudeep.holla@....com>, Wei Wang <wvw@...gle.com>,
	Jaegeuk Kim <jaegeuk@...nel.org>,
	Bart Van Assche <bvanassche@....org>,
	Christoph Hellwig <hch@...radead.org>, Jens Axboe <axboe@...nel.dk>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>
Subject: Re: [PATCH v2 0/2] sched: blk: Handle HMP systems when completing IO

On 02/24/24 19:13, Shrikanth Hegde wrote:
> 
> 
> On 2/23/24 9:27 PM, Qais Yousef wrote:
> > Due to recent changes in how topology is represented on asymmetric multi
> > processing systems like big.LITTLE where all cpus share the last LLC, there is
> > a performance regression as cpus with different compute capacities appear under
> > the same LLC and we no longer send an IPI when the requester is running on
> > a different cluster with different compute capacity.
> > 
> > Restore the old behavior by adding a new cpus_equal_capacity() function to help
> > check for the new condition for these systems.
> > 
> > Changes since v1:
> > 
> > 	* Split the patch per subsystem.
> > 	* Convert cpus_gte_capacity() to cpus_equal_capacity()
> > 	* Make cpus_equal_capacity() return immediately for SMP systems.
> > 
> 
> nit: Did you mean !SMP systems here? 
> Because in changes i see its returning true directly if its in !CONFIG_SMP path. 

I was referring to this hunk

+       if (!sched_asym_cpucap_active())
+               return true;

in cpus_equal_capacity(). In SMP system the condition is always true and
there's a static key that tells us if the system is asymmetric.

> 
> > Qais Yousef (2):
> >   sched: Add a new function to compare if two cpus have the same
> >     capacity
> >   block/blk-mq: Don't complete locally if capacities are different
> > 
> >  block/blk-mq.c                 |  5 +++--
> >  include/linux/sched/topology.h |  6 ++++++
> >  kernel/sched/core.c            | 11 +++++++++++
> >  3 files changed, 20 insertions(+), 2 deletions(-)
> >