linux-kernel - Re: [RFC 1/2] softirq: Defer net rx/tx processing to ksoftirqd context

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Message-ID: <CAFTL4hyPrwo+RXUkBp5FS_MC6qiMsK5bCVg2_tn0uPu6d_fgqg@mail.gmail.com>
Date:   Tue, 9 Jan 2018 17:03:53 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Dmitry Safonov <dima@...sta.com>
Cc:     linux-kernel@...r.kernel.org, 0x7f454c46@...il.com,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Miller <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Hannes Frederic Sowa <hannes@...essinduktion.org>,
        Ingo Molnar <mingo@...nel.org>,
        "Levin, Alexander (Sasha Levin)" <alexander.levin@...izon.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Paolo Abeni <pabeni@...hat.com>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Radu Rendec <rrendec@...sta.com>,
        Rik van Riel <riel@...hat.com>,
        Stanislaw Gruszka <sgruszka@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Wanpeng Li <wanpeng.li@...mail.com>
Subject: Re: [RFC 1/2] softirq: Defer net rx/tx processing to ksoftirqd context

2018-01-09 14:36 UTC+01:00, Dmitry Safonov <dima@...sta.com>:
> Warning: Not merge-ready
>
> I. Current workflow of ksoftirqd.
>   Softirqs are processed in the context of ksoftirqd iff they are
>   being raised very frequently. How it works:
>   do_softirq() and invoke_softirq() deffer pending softirq iff
>   ksoftirqd is in runqueue. Ksoftirqd is scheduled mostly in the
>   end of processed softirqs if 2ms were not enough to process all
>   pending softirqs.
>
>   Here is pseudo-picture of the workflow (for simplicity on UMP):
>   -------------      ------------------      ------------------
>   | ksoftirqd |      | User's process |      |   Softirqs     |
>   -------------      ------------------      ------------------
>    Not scheduled          Running
>                              |
>                              o------------------------o
>                                                       |
>                                                 __do_softirq()
>                                                       |
>                                               2ms & softirq pending?
>                                               Schedule ksoftirqd
>                                                       |
>     Scheduled                o------------------------o
>                              |
>         o--------------------o
>         |
>      Running             Scheduled
>         |
>         o--------------------o
>                              |
>    Not scheduled          Running
>
>    Timegraph for the workflow,
>      dash (-) means ksoftirqd not scheduled;
>      equal(=) ksoftirqd is scheduled, a softirq may still be pending
>
>                            Pending softirqs
>                 | | | |           | | | |       |
>                 v v v v           | | | |       v
>    Processing   o-----o           | | | |       o--o
>     softirqs    |     |           | | | |       |  |
>                 |     |           | | | |       |  |
>                 |     |           | | | |       |  |
>    Userspace  o-o     o=========o | | | |  o----o  o---------o
>                 <-2ms->         | | | | |  |
>                                 | v v v v  |
>    Ksoftirqd                    o----------o
>
> II. Corner-conditions.
>   During testing of commit [1] on some non-mainstream driver,
>   I've found that due to platform specifics, the IRQ is being
>   raised too late (after softirq has been processed).

I'm a bit confused about that part. I would expect the softirq to be
raised by the IRQ.
So I guess in this scenario the softirq is raised by something else
and you expect the upcoming IRQ to handle the softirq, right? (sorry
I'm not used to networking code).

Thanks.

>   In result softirqs steal time from userspace process, leaving
>   it starving for CPU time and never/rarely scheduling ksoftirqd: