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From:   Sandeep Dhavale <dhavale@google.com>
Date:   Thu, 13 Jul 2023 16:08:29 -0700
Message-ID: <CAB=BE-SwUTDkVvd5s3-NjEzBTqoZnHFdZg0OU-YVK+h3rxnEuw@mail.gmail.com>
Subject: Re: [PATCH v1] rcu: Fix and improve RCU read lock checks when !CONFIG_DEBUG_LOCK_ALLOC
To:     paulmck@kernel.org
Cc:     Joel Fernandes <joel@joelfernandes.org>,
        Gao Xiang <hsiangkao@linux.alibaba.com>,
        Frederic Weisbecker <frederic@kernel.org>,
        Neeraj Upadhyay <quic_neeraju@quicinc.com>,
        Josh Triplett <josh@joshtriplett.org>,
        Boqun Feng <boqun.feng@gmail.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        Zqiang <qiang.zhang1211@gmail.com>,
        Matthias Brugger <matthias.bgg@gmail.com>,
        AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@collabora.com>,
        linux-erofs@lists.ozlabs.org, xiang@kernel.org,
        Will Shiu <Will.Shiu@mediatek.com>, kernel-team@android.com,
        rcu@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org,
        linux-mediatek@lists.infradead.org
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>
> 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.

> 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.

Thanks,
Sandeep.

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