linux-kernel - Re: [PATCH v1] rcu: Fix and improve RCU read lock checks when !CONFIG_DEBUG_LOCK

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Message-ID: <906e7981-9da2-4b3d-b200-aad7c8057bef@paulmck-laptop>
Date:   Thu, 13 Jul 2023 16:28:51 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Sandeep Dhavale <dhavale@...gle.com>
Cc:     Joel Fernandes <joel@...lfernandes.org>,
        Gao Xiang <hsiangkao@...ux.alibaba.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Neeraj Upadhyay <quic_neeraju@...cinc.com>,
        Josh Triplett <josh@...htriplett.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Zqiang <qiang.zhang1211@...il.com>,
        Matthias Brugger <matthias.bgg@...il.com>,
        AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@...labora.com>,
        linux-erofs@...ts.ozlabs.org, xiang@...nel.org,
        Will Shiu <Will.Shiu@...iatek.com>, kernel-team@...roid.com,
        rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-mediatek@...ts.infradead.org
Subject: Re: [PATCH v1] rcu: Fix and improve RCU read lock checks when
 !CONFIG_DEBUG_LOCK_ALLOC

On Thu, Jul 13, 2023 at 04:08:29PM -0700, Sandeep Dhavale wrote:
> >
> > Sorry, but the current lockdep-support functions need to stay focused
> > on lockdep.  They are not set up for general use, as we already saw
> > with rcu_is_watching().
> >
> Ok, understood.
> 
> > If you get a z_erofs_wq_needed() (or whatever) upstream, and if it turns
> > out that there is an RCU-specific portion that has clean semantics,
> > then I would be willing to look at pulling that portion into RCU.
> > Note "look at" as opposed to "unconditionally agree to".  ;-)
> > > > I have no official opinion myself, but there are quite a few people
> > > ...
> > >
> > > Regarding erofs trying to detect this, I understand few people can
> > > have different
> > > opinions. Not scheduling a thread while being in a thread context itself
> > > is reasonable in my opinion which also has shown performance gains.
> >
> > You still haven't quantified the performance gains.  Presumably they
> > are most compelling with large numbers of small buffers to be decrypted.
> 
> Maybe you missed one of the replies. Link [1] shows the scheduling overhead
> for kworker vs high pri kthread. I think we can all see that there is non-zero
> cost associated with always scheduling vs inline decompression.

Heh!  A reply I was not CCed on back in February.  ;-)

But data like that included directly in the commit log, gathered
specifically for that commit log's patch, would be very helpful.

> > But why not just make a z_erofs_wq_needed() or similar in your own
> > code, and push it upstream?  If the performance gains really are so
> > compelling, one would hope that some sort of reasonable arrangement
> > could be arrived at.
> >
> Yes, we will incorporate additional checks in erofs.

Sounds good to me!

							Thanx, Paul

> Thanks,
> Sandeep.
> 
> [1] https://lore.kernel.org/linux-erofs/20230208093322.75816-1-hsiangkao@linux.alibaba.com/