linux-kernel - Re: [PATCH v5 02/11] mm/mempolicy: introduce MPOL_WEIGHTED

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Message-ID: <87mstg9uay.fsf@yhuang6-desk2.ccr.corp.intel.com>
Date: Mon, 08 Jan 2024 15:08:05 +0800
From: "Huang, Ying" <ying.huang@...el.com>
To: Gregory Price <gregory.price@...verge.com>
Cc: Gregory Price <gourry.memverge@...il.com>,  <linux-mm@...ck.org>,
  <linux-doc@...r.kernel.org>,  <linux-fsdevel@...r.kernel.org>,
  <linux-kernel@...r.kernel.org>,  <linux-api@...r.kernel.org>,
  <x86@...nel.org>,  <akpm@...ux-foundation.org>,  <arnd@...db.de>,
  <tglx@...utronix.de>,  <luto@...nel.org>,  <mingo@...hat.com>,
  <bp@...en8.de>,  <dave.hansen@...ux.intel.com>,  <hpa@...or.com>,
  <mhocko@...nel.org>,  <tj@...nel.org>,  <corbet@....net>,
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  <vtavarespetr@...ron.com>,  <peterz@...radead.org>,
  <jgroves@...ron.com>,  <ravis.opensrc@...ron.com>,
  <sthanneeru@...ron.com>,  <emirakhur@...ron.com>,  <Hasan.Maruf@....com>,
  <seungjun.ha@...sung.com>,  Srinivasulu Thanneeru
 <sthanneeru.opensrc@...ron.com>
Subject: Re: [PATCH v5 02/11] mm/mempolicy: introduce
 MPOL_WEIGHTED_INTERLEAVE for weighted interleaving

Gregory Price <gregory.price@...verge.com> writes:

> On Fri, Jan 05, 2024 at 02:51:40PM +0800, Huang, Ying wrote:
>> >
>> > So we're talking ~1MB for 1024 threads with mempolicies to avoid error
>> > conditions mid-page-allocation and to reduce the cost associated with
>> > applying weighted interleave.
>> 
>> Think about this again.  Why do we need weights array on stack?  I think
>> this is used to keep weights consistent.  If so, we don't need weights
>> array on stack.  Just use RCU to access global weights array.
>> 
>
> From the bulk allocation code:
>
> __alloc_pages_bulk(gfp, node, NULL, node_pages, NULL, page_array);
>
> This function can block. You cannot block during an RCU read context.

Yes.  You are right.  For __alloc_pages_bulk(), it should be OK to
allocate the weights array.  For weighted_interleave_nid(), we can use
RCU to avoid memory allocation in relative fast code path.

BTW, we can use nr_node_ids instead of MAX_NUMNODES if applicable.

--
Best Regards,
Huang, Ying