linux-kernel - Re: [PATCH 1/4] softirq: implement IRQ flood detection mechanism

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Message-ID: <299fb6b5-d414-2e71-1dd2-9d6e34ee1c79@linaro.org>
Date:   Tue, 3 Sep 2019 07:59:39 +0200
From:   Daniel Lezcano <daniel.lezcano@...aro.org>
To:     Ming Lei <ming.lei@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Long Li <longli@...rosoft.com>, Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Keith Busch <keith.busch@...el.com>, Jens Axboe <axboe@...com>,
        Christoph Hellwig <hch@....de>,
        Sagi Grimberg <sagi@...mberg.me>,
        John Garry <john.garry@...wei.com>,
        Hannes Reinecke <hare@...e.com>,
        linux-nvme@...ts.infradead.org, linux-scsi@...r.kernel.org
Subject: Re: [PATCH 1/4] softirq: implement IRQ flood detection mechanism


Hi Ming Lei,

On 03/09/2019 05:30, Ming Lei wrote:

[ ... ]


>>> 2) irq/timing doesn't cover softirq
>>
>> That's solvable, right?
> 
> Yeah, we can extend irq/timing, but ugly for irq/timing, since irq/timing
> focuses on hardirq predication, and softirq isn't involved in that
> purpose.
> 
>>  
>>> Daniel, could you take a look and see if irq flood detection can be
>>> implemented easily by irq/timing.c?
>>
>> I assume you can take a look as well, right?
> 
> Yeah, I have looked at the code for a while, but I think that irq/timing
> could become complicated unnecessarily for covering irq flood detection,
> meantime it is much less efficient for detecting IRQ flood.

In the series, there is nothing describing rigorously the problem (I can
only guess) and why the proposed solution solves it.

What is your definition of an 'irq flood'? A high irq load? An irq
arriving while we are processing the previous one in the bottom halves?

The patch 2/4 description says "however IO completion is only done on
one of these submission CPU cores". That describes the bottleneck and
then the patch says "Add IRQF_RESCUE_THREAD to create one interrupt
thread handler", what is the rational between the bottleneck (problem)
and the irqf_rescue_thread (solution)?

Is it really the solution to track the irq timings to detect a flood?



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