linux-kernel - Re: [RESEND][PATCH] mm: vmstats: tlb flush counters

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Date:	Fri, 19 Jul 2013 10:28:48 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Dave Hansen <dave@...1.net>, linux-kernel@...r.kernel.org,
	x86@...nel.org, linux-mm@...ck.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [RESEND][PATCH] mm: vmstats: tlb flush counters


* Andrew Morton <akpm@...ux-foundation.org> wrote:

> On Wed, 17 Jul 2013 09:21:00 +0200 Ingo Molnar <mingo@...nel.org> wrote:
> 
> > 
> > * Dave Hansen <dave@...1.net> wrote:
> > 
> > > I was investigating some TLB flush scaling issues and realized
> > > that we do not have any good methods for figuring out how many
> > > TLB flushes we are doing.
> > > 
> > > It would be nice to be able to do these in generic code, but the
> > > arch-independent calls don't explicitly specify whether we
> > > actually need to do remote flushes or not.  In the end, we really
> > > need to know if we actually _did_ global vs. local invalidations,
> > > so that leaves us with few options other than to muck with the
> > > counters from arch-specific code.
> 
> Spose so, if you really think it's worth it.  It's all downside for 
> uniprocessor machines. [...]

UP is slowly going extinct, but in any case these counters ought to inform 
us about TLB flushes even on UP systems:

> > > +		NR_TLB_LOCAL_FLUSH_ALL,
> > > +		NR_TLB_LOCAL_FLUSH_ONE,
> > > +		NR_TLB_LOCAL_FLUSH_ONE_KERNEL,

While these ought to be compiled out on UP kernels:

> > > +		NR_TLB_REMOTE_FLUSH,	/* cpu tried to flush others' tlbs */
> > > +		NR_TLB_REMOTE_FLUSH_RECEIVED,/* cpu received ipi for flush */

Right?

> > Please fix the vertical alignment of comments.
> 
> I looked - this isn't practical.
> 
> It would be nice to actually document these things though.  We don't 
> *have* to squeeze the comment into the RHS.

Agreed.

Thanks,

	Ingo
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