linux-kernel - Re: [PATCH v3 1/3] PM: domains: Make set_performance

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Message-ID: <20210119034428.qgjjlvti6sirsnzy@vireshk-i7>
Date:   Tue, 19 Jan 2021 09:14:28 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Ulf Hansson <ulf.hansson@...aro.org>
Cc:     Dmitry Osipenko <digetx@...il.com>,
        Thierry Reding <thierry.reding@...il.com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        Peter Geis <pgwipeout@...il.com>,
        Nicolas Chauvet <kwizart@...il.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Kevin Hilman <khilman@...nel.org>,
        Peter De Schrijver <pdeschrijver@...dia.com>,
        Viresh Kumar <vireshk@...nel.org>,
        Stephen Boyd <sboyd@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Matt Merhar <mattmerhar@...tonmail.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-tegra <linux-tegra@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>
Subject: Re: [PATCH v3 1/3] PM: domains: Make set_performance_state()
 callback optional

On 18-01-21, 13:46, Ulf Hansson wrote:
> You seem to be worried about latency/overhead while doing the
> propagation upwards in the hierarchy. That seems like a reasonable
> concern to me, especially as the genpd lock is taken at each level.

I am not sure how many levels of domains we have normally, but unless the number
is big it won't be a very big problem.

> However, to mitigate this can be rather messy. In principle, we would
> need to walk the hierarchy upwards, each time a new subdomain is added
> in genpd_add_subdomain(). While doing this, we would also need to keep
> track on what level we set to continue the propagation of the
> performance states for. Even if this can be done in non-latency
> sensitive paths, I don't think it's worth it because of complexity (I
> haven't even thought of what happens when removing a subdomain).

What about a new field in the domain structure like 'can-handle-pstates', and
then whenever sub-domain gets added it just needs to check its parent's field
and set his own.

> So, maybe we should simply just stick to the existing code, forcing
> the parent to have a ->set_performance() callback assigned if
> propagation should continue?

I think it would be better to fix the issue even if we aren't fully optimized
and making the change to make sure we keep propagating is rather important.

-- 
viresh