Re: [PATCH v16 18/22] mm/lru: replace pgdat lru_lock with lruvec lock

Alex Shi
2020-07-18T14:15:02Z

在 2020/7/18 上午5:38, Alexander Duyck 写道:
>> + return locked_lruvec;
>> +
>> + if (locked_lruvec)
>> + unlock_page_lruvec_irqrestore(locked_lruvec, *flags);
>> +
>> + return lock_page_lruvec_irqsave(page, flags);
>> +}
>> +
> These relock functions have no users in this patch. It might make
> sense and push this code to patch 19 in your series since that is
> where they are first used. In addition they don't seem very efficient
> as you already had to call mem_cgroup_page_lruvec once, why do it
> again when you could just store the value and lock the new lruvec if
> needed?

Right, it's better to move for late patch.

As to call the func again, mainly it's for code neat.

Thanks!

>
>> #ifdef CONFIG_CGROUP_WRITEBACK
>>
>> struct wb_domain *mem_cgroup_wb_domain(struct bdi_writeback *wb);
>> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
>> index 14c668b7e793..36c1680efd90 100644
>> --- a/include/linux/mmzone.h
>> +++ b/include/linux/mmzone.h
>> @@ -261,6 +261,8 @@ struct lruvec {
>> atomic_long_t nonresident_age;
>> /* Refaults at the time of last reclaim cycle */
>> unsigned long refaults;
>> + /* per lruvec lru_lock for memcg */
>> + spinlock_t lru_lock;
>> /* Various lruvec state flags (enum lruvec_flags) */
>> unsigned long flags;
> Any reason for placing this here instead of at the end of the
> structure? From what I can tell it looks like lruvec is already 128B
> long so placing the lock on the end would put it into the next
> cacheline which may provide some performance benefit since it is
> likely to be bounced quite a bit.

Rong Chen(Cced) once reported a performance regression when the lock at
the end of struct, and move here could remove it.
Although I can't not reproduce. But I trust his report.

...

>> putback:
>> - spin_unlock_irq(&zone->zone_pgdat->lru_lock);
>> pagevec_add(&pvec_putback, pvec->pages[i]);
>> pvec->pages[i] = NULL;
>> }
>> - /* tempary disable irq, will remove later */
>> - local_irq_disable();
>> __mod_zone_page_state(zone, NR_MLOCK, delta_munlocked);
>> - local_irq_enable();
>> + if (lruvec)
>> + unlock_page_lruvec_irq(lruvec);
> So I am not a fan of this change. You went to all the trouble of
> reducing the lock scope just to bring it back out here again. In
> addition it implies there is a path where you might try to update the
> page state without disabling interrupts.

Right. but any idea to avoid this except a extra local_irq_disable?

...

>> if (PageLRU(page)) {
>> - struct pglist_data *pgdat = page_pgdat(page);
>> + struct lruvec *new_lruvec;
>>
>> - if (pgdat != locked_pgdat) {
>> - if (locked_pgdat)
>> - spin_unlock_irqrestore(&locked_pgdat->lru_lock,
>> + new_lruvec = mem_cgroup_page_lruvec(page,
>> + page_pgdat(page));
>> + if (new_lruvec != lruvec) {
>> + if (lruvec)
>> + unlock_page_lruvec_irqrestore(lruvec,
>> flags);
>> lock_batch = 0;
>> - locked_pgdat = pgdat;
>> - spin_lock_irqsave(&locked_pgdat->lru_lock, flags);
>> + lruvec = lock_page_lruvec_irqsave(page, &flags);
>> }
> This just kind of seems ugly to me. I am not a fan of having to fetch
> the lruvec twice when you already have it in new_lruvec. I suppose it
> is fine though since you are just going to be replacing it later
> anyway.
>

yes, it will be reproduce later.

Thanks
Alex