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From: Frederic Weisbecker <frederic@kernel.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: LKML <linux-kernel@vger.kernel.org>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
	Peter Zijlstra <peterz@infradead.org>,
	"David S . Miller" <davem@davemloft.net>,
	Mauro Carvalho Chehab <mchehab+samsung@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	"Paul E . McKenney" <paulmck@linux.vnet.ibm.com>,
	Frederic Weisbecker <fweisbec@gmail.com>,
	Pavan Kondeti <pkondeti@codeaurora.org>,
	Ingo Molnar <mingo@kernel.org>,
	Joel Fernandes <joel@joelfernandes.org>
Subject: Re: [PATCH 00/37] softirq: Per vector masking v3
Date: Fri, 1 Mar 2019 04:45:37 +0100	[thread overview]
Message-ID: <20190301034536.GA19200@lenoir> (raw)
In-Reply-To: <CAHk-=wh5MQiXNUBCN6VKNGRR2gDekhXpaTc914D3y4=dHpmh4Q@mail.gmail.com>

On Thu, Feb 28, 2019 at 09:33:15AM -0800, Linus Torvalds wrote:
> On Thu, Feb 28, 2019 at 9:12 AM Frederic Weisbecker <frederic@kernel.org> wrote:
> >
> > So this set should hopefully address all reviews from the v2, and
> > fix all reports from the extremely useful (as always) Kbuild testing
> > bot. It also completes support for all archs.
> 
> The one thing I'd still like to see is some actual performance
> (latency?) numbers.
> 
> Maybe they were hiding somewhere in the pile and my quick scan missed
> them. But the main argument for this was that we've had the occasional
> latency issues with softirqs blocking (eg the USB v4l frame dropping
> etc), and I did that SOFTIRQ_NOW_MASK because it helped one particular
> case.
> 
> And you don't seem to have removed that hack, and I'd really like to
> see that that thing isn't needed any more.
> 
> Because otherwise the whole series seems a bit pointless, don't you
> think? If it doesn't fix that fundamental issue, then what's the point
> of all this churn..

Numbers are indeed missing. In fact this patchset mostly just brings an
infrastructure. We have yet to pinpoint the most latency-inducing
softirq disabled sites and make them disable only the vectors that
are involved in a given lock.

And last but not least, this patchset allows us to soft-interrupt
code that disabled other vectors but it doesn't yet allow us to
soft-interrupt a vector itself. Not much is needed to allow that
from the softirq core code. But we can't do that blindly. For example
TIMER_SOFTIRQ, HRTIMER_SOFTIRQ, TASKLET_SOFTIRQ, NET_RX_SOFTIRQ
can't interrupt each others because some locks can be taken on all
of them (the socket lock for example). Although so many vectors
involved for a single lock is probably rare but still...

The only solution I see to make vectors interruptible is to proceed
the same way as we do for softirq disabled sections: proceed case
by case on a per handler basis. Hopefully we can operate per subsystem
and we don't need to start from drivers.

So the idea is the following: if the lock A can be taken from both TIMER_SOFTIRQ
and BLOCK_SOFTIRQ, we do this from the timer handler for example:

         __do_softirq() {
	     // all vectors disabled
	     run_timers {
	         random_timer_callback() {
                     bh = local_bh_enable_mask(~(TIMER_SOFTIRQ | BLOCK_SOFTIRQ));
                     spin_lock(&A);
                     do_some_work();
                     spin_unlock(&A);
                     local_bh_disable_mask(bh);
		 }
            }
         }

Sounds tedious but that's the only way I can imagine to make that correct.

Another way could be for locks to piggyback the vectors they are involved in
on initialization:

DEFINE_SPINLOCK_SOFTIRQ(A, TIMER_SOFTIRQ | BLOCK_SOFTIRQ);

Then callsites can just use:

    bh = spin_lock_softirq(A);
    ....
    spin_unlock_softirq(A, bh);

Then the lock function always arrange to only disable TIMER_SOFTIRQ | BLOCK_SOFTIRQ
if not nesting, whether we are in a vector or not. The only drawback is for the
relevant spin_lock_t to carry those init flags.

> 
> See commit 3c53776e29f8 ("Mark HI and TASKLET softirq synchronous"),
> which also has a couple of people listed who could hopefully re-test
> the v4l latency thing with whatever USB capture dongle it was that
> showed the issue.

So in this case for example, I'll need to check the callbacks involved
and make them disable only the vectors that need to be disabled.

I should try to reproduce the issue myself.

Thanks.

  reply	other threads:[~2019-03-01  3:46 UTC|newest]

Thread overview: 44+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-02-28 17:12 Frederic Weisbecker
2019-02-28 17:12 ` [PATCH 01/37] locking/lockdep: Move valid_state() inside CONFIG_TRACE_IRQFLAGS && CONFIG_PROVE_LOCKING Frederic Weisbecker
2019-02-28 17:12 ` [PATCH 02/37] locking/lockdep: Use expanded masks on find_usage_*() functions Frederic Weisbecker
2019-02-28 17:12 ` [PATCH 03/37] locking/lockdep: Introduce struct lock_usage Frederic Weisbecker
2019-02-28 17:12 ` [PATCH 04/37] locking/lockdep: Convert usage_mask to u64 Frederic Weisbecker
2019-02-28 17:12 ` [PATCH 05/37] locking/lockdep: Introduce lock usage mask iterator Frederic Weisbecker
2019-02-28 17:12 ` [PATCH 06/37] locking/lockdep: Test all incompatible scenario at once in check_irq_usage() Frederic Weisbecker
2019-02-28 17:12 ` [PATCH 07/37] locking/lockdep: Prepare valid_state() to handle plain masks Frederic Weisbecker
2019-02-28 17:12 ` [PATCH 08/37] locking/lockdep: Prepare check_usage_*() " Frederic Weisbecker
2019-02-28 17:12 ` [PATCH 09/37] locking/lockdep: Prepare state_verbose() to handle all softirqs Frederic Weisbecker
2019-02-28 17:12 ` [PATCH 10/37] locking/lockdep: Make mark_lock() fastpath to work with multiple usage at once Frederic Weisbecker
2019-02-28 17:12 ` [PATCH 11/37] locking/lockdep: Save stack trace for each softirq vector involved Frederic Weisbecker
2019-02-28 17:12 ` [PATCH 12/37] locking/lockdep: Report all usages on mark_lock() verbosity mode Frederic Weisbecker
2019-02-28 17:12 ` [PATCH 13/37] softirq: Macrofy softirq vectors Frederic Weisbecker
2019-02-28 17:12 ` [PATCH 14/37] locking/lockdep: Define per vector softirq lock usage states Frederic Weisbecker
2019-04-09 12:03   ` Peter Zijlstra
2019-02-28 17:12 ` [PATCH 15/37] softirq: Pass softirq vector number to lockdep on vector execution Frederic Weisbecker
2019-02-28 17:12 ` [PATCH 16/37] x86: Revert "x86/irq: Demote irq_cpustat_t::__softirq_pending to u16" Frederic Weisbecker
2019-02-28 17:12 ` [PATCH 17/37] arch/softirq: Rename softirq_pending fields to softirq_data Frederic Weisbecker
2019-02-28 17:12 ` [PATCH 18/37] softirq: Normalize softirq_pending naming scheme Frederic Weisbecker
2019-02-28 17:12 ` [PATCH 19/37] softirq: Convert softirq_pending_*() to set/clear mask scheme Frederic Weisbecker
2019-02-28 17:12 ` [PATCH 20/37] softirq: Introduce disabled softirq vectors bits Frederic Weisbecker
2019-03-01 11:29   ` Sebastian Andrzej Siewior
2019-02-28 17:12 ` [PATCH 21/37] softirq: Rename _local_bh_enable() to local_bh_enable_no_softirq() Frederic Weisbecker
2019-02-28 17:12 ` [PATCH 22/37] softirq: Move vectors bits to bottom_half.h Frederic Weisbecker
2019-02-28 17:12 ` [PATCH 23/37] x86: Init softirq enabled field Frederic Weisbecker
2019-02-28 17:12 ` [PATCH 24/37] parisc: " Frederic Weisbecker
2019-02-28 17:12 ` [PATCH 25/37] powerpc: " Frederic Weisbecker
2019-02-28 17:12 ` [PATCH 26/37] softirq: Init softirq enabled field for default irq_stat definition Frederic Weisbecker
2019-02-28 17:12 ` [PATCH 27/37] softirq: Check enabled vectors before processing Frederic Weisbecker
2019-02-28 17:12 ` [PATCH 28/37] softirq: Remove stale comment Frederic Weisbecker
2019-02-28 17:12 ` [PATCH 29/37] softirq: Uninline !CONFIG_TRACE_IRQFLAGS __local_bh_disable_ip() Frederic Weisbecker
2019-02-28 17:12 ` [PATCH 30/37] softirq: Prepare for mixing all/per-vector masking Frederic Weisbecker
2019-02-28 17:12 ` [PATCH 31/37] softirq: Support per vector masking Frederic Weisbecker
2019-02-28 17:12 ` [PATCH 32/37] locking/lockdep: Remove redundant softirqs on check Frederic Weisbecker
2019-02-28 17:12 ` [PATCH 33/37] locking/lockdep: Update check_flags() according to new layout Frederic Weisbecker
2019-02-28 17:12 ` [PATCH 34/37] locking/lockdep: Branch the new vec-finegrained softirq masking to lockdep Frederic Weisbecker
2019-02-28 17:12 ` [PATCH 35/37] softirq: Allow to soft interrupt vector-specific masked contexts Frederic Weisbecker
2019-02-28 17:12 ` [PATCH 36/37] locking: Introduce spin_[un]lock_bh_mask() Frederic Weisbecker
2019-02-28 17:12 ` [PATCH 37/37] net: Make softirq vector masking finegrained on release_sock() Frederic Weisbecker
2019-02-28 17:33 ` [PATCH 00/37] softirq: Per vector masking v3 Linus Torvalds
2019-03-01  3:45   ` Frederic Weisbecker [this message]
2019-03-01 16:51     ` Linus Torvalds
2019-03-08 15:30       ` Frederic Weisbecker

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