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

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Message-ID: <1521124920.2686.20.camel@arista.com>
Date:   Thu, 15 Mar 2018 14:42:00 +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 14:34 +0000, Dmitry Safonov wrote:
> 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..
> 

So I agree, that maybe better to have another limit to the cycle
*also*, because if we clean faults with the same speed as they're
generated by hw, we may stuck in the loop..
By on my measures clearing fault is so fast (< 1 usec), that I'm not
sure that it may happen with hw. By that reason I didn't introduce
loop-limit.

But even with loop-limit we will need ratelimit each printk() *also*.
Otherwise loop-limit will be based on time spent printing, not on
anything else..
The patch makes sense even with loop-limit in my opinion.

-- 
Thanks,
             Dmitry