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Date:   Tue, 5 Dec 2023 16:31:46 +0100
From:   Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To:     Yuanhan Zhang <zyhtheonly@gmail.com>
Cc:     zyhtheonly@yeah.net, tglx@linutronix.de, rostedt@goodmis.org,
        mingo@redhat.com, Venkatesh Pallipadi <venki@google.com>,
        peterz@infradead.org, juri.lelli@redhat.com,
        vincent.guittot@linaro.org, linux-kernel@vger.kernel.org,
        dietmar.eggemann@arm.com, bsegall@google.com, mgorman@suse.de,
        bristot@redhat.com, vschneid@redhat.com
Subject: Re: [PATCH v3] sched/cputime: let ktimers align with ksoftirqd in
 accounting CPUTIME_SOFTIRQ
Message-ID: <20231205153146.OSpCIs1G@linutronix.de>
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On 2023-12-02 05:28:15 [-0500], Yuanhan Zhang wrote:
> Hi,
Hi,

> Sebastian Andrzej Siewior <bigeasy@linutronix.de> =E4=BA=8E2023=E5=B9=B41=
2=E6=9C=881=E6=97=A5=E5=91=A8=E4=BA=94 11:16=E5=86=99=E9=81=93=EF=BC=9A
> >
> > On 2023-12-01 16:05:41 [+0800], tiozhang wrote:
> > > In CONFIG_PREEMPT_RT kernel, ktimers also calls __do_softirq,
> > > so when accounting CPUTIME_SOFTIRQ, ktimers need to be accounted the =
same
> > > as ksoftirqd.
> >
> > I still don't understand why this is a good thing and why want to align
> > it with ksoftirqd and what breaks if we don't.
>=20
> My motivation of doing this is to keep CPUTIME_SOFTIRQ in /proc/stat
> remaining more accurate in PREEPT_RT kernel.
>=20
> If we dont align, ktimers' cpu time is added to CPUTIME_SYSTEM when
> CONFIG_IRQ_TIME_ACCOUNTING is enabled, make our stats less accurate..

So it is only SYSTEM? There is no additional SOFTIRQ that is used?

> The main diff between ksoftirqd and force-threaded interrupt is that ksof=
tirq
> is in SOFTIRQ_OFFSET (serving softirqs) while  force-threaded is in
> SOFTIRQ_DISABLE_OFFSET (by using local_disable_bh).

This depends. If you look at a network driver, SOFTIRQ_DISABLE_OFFSET is
used during the driver routine doing almost only schedule a softirq.
Then the main part happens during SOFTIRQ_OFFSET where the driver pulls
the packets and passes them to the network stack.

> CPUTIME_SOFTIRQ serves for time in SOFTIRQ_OFFSET (processing softirqs).
> See
> https://lore.kernel.org/all/1285619753-10892-1-git-send-email-venki@googl=
e.com/

Let me look at this later=E2=80=A6

> So this leads to ksoftirqd is counted into CPUTIME_SOFTIRQ but irq-threads
> into CPUTIME_SYSTEM.
>=20
> Since ktimers is also in SOFTIRQ_OFFSET, align it with ksoftirq will
> put it into CPUTIME_SOFTIRQ, making /proc/stat more accurate.

But this only works if it is observed during the TICK interrupt, right?

Sebastian