On 11/5/20 9:55 AM, Alex Shi wrote:
This patch moves per node lru_lock into lruvec, thus bring a lru_lock for
each of memcg per node. So on a large machine, each of memcg don't
have to suffer from per node pgdat->lru_lock competition. They could go
fast with their self lru_lock.
After move memcg charge before lru inserting, page isolation could
serialize page's memcg, then per memcg lruvec lock is stable and could
replace per node lru lock.
In func isolate_migratepages_block, compact_unlock_should_abort and
lock_page_lruvec_irqsave are open coded to work with compact_control.
Also add a debug func in locking which may give some clues if there are
sth out of hands.
Daniel Jordan's testing show 62% improvement on modified readtwice case
on his 2P * 10 core * 2 HT broadwell box.
https://lore.kernel.org/lkml/20200915165807.kpp7uhiw7l3loofu@xxxxxxxxxxxxxxxxxxxxxxxxxx/
On a large machine with memcg enabled but not used, the page's lruvec
seeking pass a few pointers, that may lead to lru_lock holding time
increase and a bit regression.
Hugh Dickins helped on the patch polish, thanks!
Signed-off-by: Alex Shi <alex.shi@xxxxxxxxxxxxxxxxx>
Acked-by: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Rong Chen <rong.a.chen@xxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxxxx>
Cc: Vladimir Davydov <vdavydov.dev@xxxxxxxxx>
Cc: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>
Cc: Tejun Heo <tj@xxxxxxxxxx>
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: linux-mm@xxxxxxxxx
Cc: cgroups@xxxxxxxxxxxxxxx
I think I need some explanation about the rcu_read_lock() usage in
lock_page_lruvec*() (and places effectively opencoding it).
Preferably in form of some code comment, but that can be also added as a
additional patch later, I don't want to block the series.
mem_cgroup_page_lruvec() comment says
* This function relies on page->mem_cgroup being stable - see the
* access rules in commit_charge().
commit_charge() comment:
* Any of the following ensures page->mem_cgroup stability:
*
* - the page lock
* - LRU isolation
* - lock_page_memcg()
* - exclusive reference
"LRU isolation" used to be quite clear, but now is it after
TestClearPageLRU(page) or after deleting from the lru list as well?
Also it doesn't mention rcu_read_lock(), should it?
So what exactly are we protecting by rcu_read_lock() in e.g. lock_page_lruvec()?
rcu_read_lock();
lruvec = mem_cgroup_page_lruvec(page, pgdat);
spin_lock(&lruvec->lru_lock);
rcu_read_unlock();
Looks like we are protecting the lruvec from going away and it can't go away
anymore after we take the lru_lock?
But then e.g. in __munlock_pagevec() we are doing this without an rcu_read_lock():
new_lruvec = mem_cgroup_page_lruvec(page, page_pgdat(page));
where new_lruvec is potentionally not the one that we have locked
And the last thing mem_cgroup_page_lruvec() is doing is:
if (unlikely(lruvec->pgdat != pgdat))
lruvec->pgdat = pgdat;
return lruvec;
So without the rcu_read_lock() is this potentionally accessing the pgdat field
of lruvec that might have just gone away?
Thanks,
Vlastimil
