linux-kernel - Re: [PATCHv3] iommu/intel: Ratelimit each dmar fault printing

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Message-ID: <1521124490.2686.16.camel@arista.com>
Date:   Thu, 15 Mar 2018 14:34:50 +0000
From:   Dmitry Safonov <dima@...sta.com>
To:     Joerg Roedel <joro@...tes.org>
Cc:     linux-kernel@...r.kernel.org, 0x7f454c46@...il.com,
        Alex Williamson <alex.williamson@...hat.com>,
        David Woodhouse <dwmw2@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        iommu@...ts.linux-foundation.org
Subject: Re: [PATCHv3] iommu/intel: Ratelimit each dmar fault printing

On Thu, 2018-03-15 at 15:22 +0100, Joerg Roedel wrote:
> On Thu, Mar 15, 2018 at 02:13:03PM +0000, Dmitry Safonov wrote:
> > So, you suggest to remove ratelimit at all?
> > Do we really need printk flood for each happened fault?
> > Imagine, you've hundreds of mappings and then PCI link flapped..
> > Wouldn't it be better to keep ratelimiting?
> > I don't mind, just it looks a bit strange to me.
> 
> I never said you should remove the ratelimiting, after all you are
> trying to fix a soft-lockup, no?
> 
> And that should not be fixed by changes to the ratelimiting, but with
> proper irq handling.

Uh, I'm a bit confused then.
- Isn't it better to ratelimit each printk() instead of bunch of
printks inside irq handler?
- I can limit the number of loops, but the most of the time is spent in
the loop on printk() (on my machine ~170msec per loop), while
everything else takes much lesser time (on my machine < 1 usec per
loop). So, if I will limit number of loops per-irq, that cycle-limit
will be based on limiting time spent on printk (e.g., how many printks
to do in atomic context so that node will not lockup). It smells like
ratelimiting, no?

I must be misunderstanding something, but why introducing another limit
for number of printk() called when there is ratelimit which may be
tuned..

-- 
Thanks,
             Dmitry