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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, 8 Mar 2019 16:30:23 +0100	[thread overview]
Message-ID: <20190308153022.GA23080@lenoir> (raw)
In-Reply-To: <CAHk-=wgOZuGZaVOOiC=drG6ykVkOGk8RRXZ_CrPBMXHKjTg0dg@mail.gmail.com>

On Fri, Mar 01, 2019 at 08:51:38AM -0800, Linus Torvalds wrote:
> On Thu, Feb 28, 2019 at 7:45 PM Frederic Weisbecker <frederic@kernel.org> wrote:
> >
> > 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.
> 
> Note that I think we pretty much know that already: the people who
> have had issues have never actually really had issues with the actual
> "disable softirq" paths, they've all been about actually *running* the
> softirq's (and that in turn being a latency issue for running _other_
> softirqs, and in particular for delaying them into softirqd).
> 
> Now, it may well be that yes, we'll have "block softirqs" code that
> has issues too, but it's absolutely been swamped by the latencies for
> actually running them so far.
> 
> Note that this is all really fairly independent of the whole masking
> logic. Yes, the masking logic comes into play too (allowing you to run
> a subset of softirq's at a time), but on the whole the complaints I've
> seen have not been "the networking softirq takes so long that it
> delays USB tasklet handling", but they have been along the lines of
> "the networking softirq gets invoked so often that it then floods the
> system and triggers softirqd, and _that_ then makes tasklet handling
> latency go up insanely".
> 
> See the difference? Not the latency of softirq's disabled, but the
> latency of one group of softirqs causing problems for another when
> they all get batched together (and soft-scheduled to another context
> together).

I see, so that's an entirely different problem that vector
soft-interruptibility can't fix, at least not alone.

The only solution I can imagine is to have a seperate pending mask
for normal softirq processing and ksoftirqd, so that only vectors
that have been enqueued for threaded processing are delayed.

I can work on that first, but I really need to be able to reproduce
an example of the issue. The USB capture thing seems to be one the best.
Let's browse some history to see if I can find some details on the
relevant scenario.

      reply	other threads:[~2019-03-08 15:30 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
2019-03-01 16:51     ` Linus Torvalds
2019-03-08 15:30       ` Frederic Weisbecker [this message]

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