Re: [PATCH v6 6/9] mm: multigenerational lru: aging

[Michal Hocko]

On Wed 19-01-22 00:04:10, Yu Zhao wrote:
> On Mon, Jan 10, 2022 at 11:54:42AM +0100, Michal Hocko wrote:
> > On Sun 09-01-22 21:47:57, Yu Zhao wrote:
> > > On Fri, Jan 07, 2022 at 03:44:50PM +0100, Michal Hocko wrote:
> > > > On Tue 04-01-22 13:22:25, Yu Zhao wrote:
> > > > [...]
> > > > > +static void walk_mm(struct lruvec *lruvec, struct mm_struct *mm, struct lru_gen_mm_walk *walk)
> > > > > +{
> > > > > +	static const struct mm_walk_ops mm_walk_ops = {
> > > > > +		.test_walk = should_skip_vma,
> > > > > +		.p4d_entry = walk_pud_range,
> > > > > +	};
> > > > > +
> > > > > +	int err;
> > > > > +#ifdef CONFIG_MEMCG
> > > > > +	struct mem_cgroup *memcg = lruvec_memcg(lruvec);
> > > > > +#endif
> > > > > +
> > > > > +	walk->next_addr = FIRST_USER_ADDRESS;
> > > > > +
> > > > > +	do {
> > > > > +		unsigned long start = walk->next_addr;
> > > > > +		unsigned long end = mm->highest_vm_end;
> > > > > +
> > > > > +		err = -EBUSY;
> > > > > +
> > > > > +		rcu_read_lock();
> > > > > +#ifdef CONFIG_MEMCG
> > > > > +		if (memcg && atomic_read(&memcg->moving_account))
> > > > > +			goto contended;
> > > > > +#endif
> > > > > +		if (!mmap_read_trylock(mm))
> > > > > +			goto contended;
> > > > 
> > > > Have you evaluated the behavior under mmap_sem contention? I mean what
> > > > would be an effect of some mms being excluded from the walk? This path
> > > > is called from direct reclaim and we do allocate with exclusive mmap_sem
> > > > IIRC and the trylock can fail in a presence of pending writer if I am
> > > > not mistaken so even the read lock holder (e.g. an allocation from the #PF)
> > > > can bypass the walk.
> > > 
> > > You are right. Here it must be a trylock; otherwise it can deadlock.
> > 
> > Yeah, this is clear.
> > 
> > > I think there might be a misunderstanding: the aging doesn't
> > > exclusively rely on page table walks to gather the accessed bit. It
> > > prefers page table walks but it can also fallback to the rmap-based
> > > function, i.e., lru_gen_look_around(), which only gathers the accessed
> > > bit from at most 64 PTEs and therefore is less efficient. But it still
> > > retains about 80% of the performance gains.
> > 
> > I have to say that I really have hard time to understand the runtime
> > behavior depending on that interaction. How does the reclaim behave when
> > the virtual scan is enabled, partially enabled and almost completely
> > disabled due to different constrains? I do not see any such an
> > evaluation described in changelogs and I consider this to be a rather
> > important information to judge the overall behavior.
> 
> It doesn't have (partially) enabled/disabled states nor does its
> behavior change with different reclaim constraints. Having either
> would make its design too complex to implement or benchmark.

Let me clarify. By "partially enabled" I really meant behavior depedning
on runtime conditions. Say mmap_sem cannot be locked for half of scanned
tasks and/or allocation for the mm walker fails due to lack of memory.
How does this going to affect reclaim efficiency. How does a user/admin
know that the memory reclaim is in a "degraded" mode because of the
contention?
-- 
Michal Hocko
SUSE Labs