Re: [PATCH 1/1] mm: add swapiness= arg to memory.reclaim

Huan Yang
2023-12-01T02:14:07Z

在 2023/12/1 10:05, Yosry Ahmed 写道:
>> @@ -2327,7 +2330,8 @@ static void get_scan_count(struct lruvec *lruvec, struct scan_control *sc,
>> struct pglist_data *pgdat = lruvec_pgdat(lruvec);
>> struct mem_cgroup *memcg = lruvec_memcg(lruvec);
>> unsigned long anon_cost, file_cost, total_cost;
>> - int swappiness = mem_cgroup_swappiness(memcg);
>> + int swappiness = sc->swappiness ?
>> + *sc->swappiness : mem_cgroup_swappiness(memcg);
>>
>> Should we use "unlikely" here to indicate that sc->swappiness is an unexpected behavior?
>> Due to current use case only apply in proactive reclaim.
> On a system that is not under memory pressure, the rate of proactive
> reclaim could be higher than reactive reclaim. We should only use
> likely/unlikely when it's obvious a scenario will happen most of the
> time. I don't believe that's the case here.
Not all vendors will use proactive interfaces, and reactive reclaim are
a normal
system behavior. In this regard, I think it is appropriate to add
"unlikely".
>
>> u64 fraction[ANON_AND_FILE];
>> u64 denominator = 0; /* gcc */
>> enum scan_balance scan_balance;
>> @@ -2608,6 +2612,9 @@ static int get_swappiness(struct lruvec *lruvec, struct scan_control *sc)
>> mem_cgroup_get_nr_swap_pages(memcg) < MIN_LRU_BATCH)
>> return 0;
>>
>> + if (sc->swappiness)
>> + return *sc->swappiness;
>>
>> Also there.
>>
>> +
>> return mem_cgroup_swappiness(memcg);
>> }
>>
>> @@ -6433,7 +6440,8 @@ unsigned long mem_cgroup_shrink_node(struct mem_cgroup *memcg,
>> unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *memcg,
>> unsigned long nr_pages,
>> gfp_t gfp_mask,
>> - unsigned int reclaim_options)
>> + unsigned int reclaim_options,
>> + int *swappiness)
>> {
>> unsigned long nr_reclaimed;
>> unsigned int noreclaim_flag;
>> @@ -6448,6 +6456,7 @@ unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *memcg,
>> .may_unmap = 1,
>> .may_swap = !!(reclaim_options & MEMCG_RECLAIM_MAY_SWAP),
>> .proactive = !!(reclaim_options & MEMCG_RECLAIM_PROACTIVE),
>> + .swappiness = swappiness,
>> };
>> /*
>> * Traverse the ZONELIST_FALLBACK zonelist of the current node to put
>> --
>> 2.34.1
>>
>> My previous patch attempted to ensure fully deterministic semantics under extreme swappiness.
>> For example, when swappiness is set to 200, only anonymous pages will be reclaimed.
>> Due to code in MGLRU isolate_folios will try scan anon if no scanned, will try other type.(We do not want
>> it to attempt this behavior.)
>> How do you think about extreme swappiness scenarios?
> I think having different semantics between swappiness passed to
> proactive reclaim and global swappiness can be confusing. If it's
> needed to have a swappiness value that says "anon only no matter
> what", perhaps we should introduce such a new value and make it
> supported by both global and proactive reclaim swappiness? We could
> support writing "max" or something similar instead of a special value
> to mean that.

Yes, use other hint more suitable for this scenario.

However, from this patch, it seems that this feature is not supported.
Do you have a demand for this scenario?

>
> Writing such value to global swappiness may cause problems and
> premature OOMs IIUC, but that would be misconfiguration. If we think
> that's dangerous, we can introduce this new value but make it valid
> only for proactive reclaim for now.