linux-kernel - Re: [PATCH] x86/mm: use max memory block size with unaligned memory end

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Date:   Thu, 4 Jun 2020 14:12:01 -0400
From:   Daniel Jordan <daniel.m.jordan@...cle.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Daniel Jordan <daniel.m.jordan@...cle.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Michal Hocko <mhocko@...nel.org>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Sistare <steven.sistare@...cle.com>
Subject: Re: [PATCH] x86/mm: use max memory block size with unaligned memory
 end

On Thu, Jun 04, 2020 at 07:45:40PM +0200, David Hildenbrand wrote:
> On 04.06.20 19:22, Daniel Jordan wrote:
> > IMHO the root cause of this is really the small block size.  Building a cache
> > on top to avoid iterating over tons of small blocks seems like papering over
> > the problem, especially when one of the two affected paths in boot is a
> 
> The memory block size dictates your memory hot(un)plug granularity.

Indeed.

> E.g., on powerpc that's 16MB so they have *a lot* of memory blocks.
> That's why that's not papering over the problem. Increasing the memory
> block size isn't always the answer.

Ok.  If you don't mind, what's the purpose of hotplugging at that granularity?
I'm simply curious.

> > cautious check that might be ready to be removed by now[0]:
> 
> Yeah, we discussed that somewhere already. My change only highlighted
> the problem. And now that it's cheap, it can just stay unless there is a
> very good reason not to do it.

Agreed.

> > Yeah, but of course it's not as bad as it was now that it's fully parallelized.
> 
> Right. I also observed that computing if a zone is contiguous can be
> expensive.

That's right, I remember that.  It's on my list :)