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Message-ID: <Zbn6FG3346jhrQga@memverge.com>
Date: Wed, 31 Jan 2024 02:43:16 -0500
From: Gregory Price <gregory.price@...verge.com>
To: "Huang, Ying" <ying.huang@...el.com>
Cc: Gregory Price <gourry.memverge@...il.com>, linux-mm@...ck.org,
linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
linux-fsdevel@...r.kernel.org, linux-api@...r.kernel.org,
corbet@....net, akpm@...ux-foundation.org, honggyu.kim@...com,
rakie.kim@...com, hyeongtak.ji@...com, mhocko@...nel.org,
vtavarespetr@...ron.com, jgroves@...ron.com,
ravis.opensrc@...ron.com, sthanneeru@...ron.com,
emirakhur@...ron.com, Hasan.Maruf@....com, seungjun.ha@...sung.com,
hannes@...xchg.org, dan.j.williams@...el.com,
Srinivasulu Thanneeru <sthanneeru.opensrc@...ron.com>
Subject: Re: [PATCH v4 3/3] mm/mempolicy: introduce MPOL_WEIGHTED_INTERLEAVE
for weighted interleaving
On Wed, Jan 31, 2024 at 02:43:12PM +0800, Huang, Ying wrote:
> Gregory Price <gourry.memverge@...il.com> writes:
> >
> > +static unsigned int weighted_interleave_nodes(struct mempolicy *policy)
> > +{
> > + unsigned int node = current->il_prev;
> > +
> > + if (!current->il_weight || !node_isset(node, policy->nodes)) {
> > + node = next_node_in(node, policy->nodes);
> > + /* can only happen if nodemask is being rebound */
> > + if (node == MAX_NUMNODES)
> > + return node;
>
> I feel a little unsafe to read policy->nodes at same time of writing in
> rebound. Is it better to use a seqlock to guarantee its consistency?
> It's unnecessary to be a part of this series though.
>
I think this is handled already? It is definitely an explicit race
condition that is documented elsewhere:
/*
* mpol_rebind_policy - Migrate a policy to a different set of nodes
*
* Per-vma policies are protected by mmap_lock. Allocations using per-task
* policies are protected by task->mems_allowed_seq to prevent a premature
* OOM/allocation failure due to parallel nodemask modification.
*/
example from slub:
do {
cpuset_mems_cookie = read_mems_allowed_begin();
zonelist = node_zonelist(mempolicy_slab_node(), pc->flags);
...
} while (read_mems_allowed_retry(cpuset_mems_cookie));
quick perusal through other allocators, show similar checks.
page_alloc.c - check_retry_cpusetset()
filemap.c - filemap_alloc_folio()
If we ever want mempolicy to be swappable from outside the current task
context, this will have to change most likely - but that's another
feature for another day.
> > + while (target) {
> > + /* detect system default usage */
> > + weight = table ? table[nid] : 1;
> > + weight = weight ? weight : 1;
>
> I found duplicated pattern as above in this patch. Can we define a
> function like below to remove the duplication?
>
> u8 __get_il_weight(u8 *table, int nid)
> {
> u8 weight;
>
> weight = table ? table[nid] : 1;
> return weight ? : 1;
> }
>
When we implement the system-default array, this will change to:
weight = sysfs_table ? sysfs_table[nid] : default_table[nid];
This cleanup will get picked up in that patch set since this code is
going to change anyway.
> > + if (delta == weight) {
> > + /* boundary: resume from next node/weight */
> > + resume_node = next_node_in(node, nodes);
> > + resume_weight = weights[resume_node];
> > + } else {
> > + /* remainder: resume this node w/ remainder */
> > + resume_node = node;
> > + resume_weight = weight - delta;
> > + }
>
> If we are comfortable to leave resume_weight == 0, then the above
> branch can be simplified to.
>
> resume_node = node;
> resume_weight = weight - delta;
>
> But, this is a style issue again. I will leave it to you to decide.
Good point, and in fact there's a similar branch in the first half of
the function that can be simplified. Will follow up with a style patch.
mm/mempolicy.c | 21 ++++-----------------
1 file changed, 4 insertions(+), 17 deletions(-)
My favorite style of patch :D
Andrew if you happen to be monitoring, this is the patch (not tested
yet, but it's pretty obvious, otherwise i'll submit individually
tomorrow).
diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 2c1aef8eab70..b0ca9bcdd64c 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -2405,15 +2405,9 @@ static unsigned long alloc_pages_bulk_array_weighted_interleave(gfp_t gfp,
page_array += nr_allocated;
total_allocated += nr_allocated;
/* if that's all the pages, no need to interleave */
- if (rem_pages < weight) {
- /* stay on current node, adjust il_weight */
+ if (rem_pages <= weight) {
me->il_weight -= rem_pages;
return total_allocated;
- } else if (rem_pages == weight) {
- /* move to next node / weight */
- me->il_prev = next_node_in(node, nodes);
- me->il_weight = get_il_weight(me->il_prev);
- return total_allocated;
}
/* Otherwise we adjust remaining pages, continue from there */
rem_pages -= weight;
@@ -2460,17 +2454,10 @@ static unsigned long alloc_pages_bulk_array_weighted_interleave(gfp_t gfp,
node_pages += weight;
delta -= weight;
} else if (delta) {
+ /* when delta is deleted, resume from that node */
node_pages += delta;
- /* delta may deplete on a boundary or w/ a remainder */
- if (delta == weight) {
- /* boundary: resume from next node/weight */
- resume_node = next_node_in(node, nodes);
- resume_weight = weights[resume_node];
- } else {
- /* remainder: resume this node w/ remainder */
- resume_node = node;
- resume_weight = weight - delta;
- }
+ resume_node = node;
+ resume_weight = weight - delta;
delta = 0;
}
/* node_pages can be 0 if an allocation fails and rounds == 0 */
>
> So, except the issue you pointed out already. All series looks good to
> me! Thanks! Feel free to add
>
> Reviewed-by: "Huang, Ying" <ying.huang@...el.com>
>
> to the whole series.
>
Thank you so much for your patience with me! I appreciate all the help.
I am looking forward to this feature very much!
~Gregory
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