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From: "Huang, Ying" <ying.huang@intel.com>
To: Gregory Price <gregory.price@memverge.com>
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Subject: Re: [PATCH v5 02/11] mm/mempolicy: introduce
 MPOL_WEIGHTED_INTERLEAVE for weighted interleaving
In-Reply-To: <ZZeu6DwVt6o0fl14@memverge.com> (Gregory Price's message of "Fri,
	5 Jan 2024 02:25:28 -0500")
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Date: Mon, 08 Jan 2024 15:08:05 +0800
Message-ID: <87mstg9uay.fsf@yhuang6-desk2.ccr.corp.intel.com>
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Gregory Price <gregory.price@memverge.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