Message-ID: <20170113091009.GD25212@dhcp22.suse.cz>
Date:   Fri, 13 Jan 2017 10:10:09 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Minchan Kim <minchan@...nel.org>
Cc:     linux-kernel@...r.kernel.org, hillf.zj@...baba-inc.com,
        mgorman@...e.de, vbabka@...e.cz, mm-commits@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: + mm-vmscan-add-mm_vmscan_inactive_list_is_low-tracepoint.patch
 added to -mm tree

On Fri 13-01-17 17:57:34, Minchan Kim wrote:
> On Fri, Jan 13, 2017 at 08:47:07AM +0100, Michal Hocko wrote:
> > On Fri 13-01-17 10:37:24, Minchan Kim wrote:
> > > Hello,
> > > 
> > > On Thu, Jan 12, 2017 at 10:10:17AM +0100, Michal Hocko wrote:
> > > > On Thu 12-01-17 17:48:13, Minchan Kim wrote:
> > > > > On Thu, Jan 12, 2017 at 09:15:54AM +0100, Michal Hocko wrote:
> > > > > > On Thu 12-01-17 14:12:47, Minchan Kim wrote:
> > > > > > > Hello,
> > > > > > > 
> > > > > > > On Wed, Jan 11, 2017 at 04:52:39PM +0100, Michal Hocko wrote:
> > > > > > > > On Wed 11-01-17 08:52:50, Minchan Kim wrote:
> > > > > > > > [...]
> > > > > > > > > > @@ -2055,8 +2055,8 @@ static bool inactive_list_is_low(struct
> > > > > > > > > >  	if (!file && !total_swap_pages)
> > > > > > > > > >  		return false;
> > > > > > > > > >  
> > > > > > > > > > -	inactive = lruvec_lru_size(lruvec, file * LRU_FILE);
> > > > > > > > > > -	active = lruvec_lru_size(lruvec, file * LRU_FILE + LRU_ACTIVE);
> > > > > > > > > > +	total_inactive = inactive = lruvec_lru_size(lruvec, file * LRU_FILE);
> > > > > > > > > > +	total_active = active = lruvec_lru_size(lruvec, file * LRU_FILE + LRU_ACTIVE);
> > > > > > > > > >  
> > > > > > > > > 
> > > > > > > > > the decision of deactivating is based on eligible zone's LRU size,
> > > > > > > > > not whole zone so why should we need to get a trace of all zones's LRU?
> > > > > > > > 
> > > > > > > > Strictly speaking, the total_ counters are not necessary for making the
> > > > > > > > decision. I found reporting those numbers useful regardless because this
> > > > > > > > will give us also an information how large is the eligible portion of
> > > > > > > > the LRU list. We do not have any other tracepoint which would report
> > > > > > > > that.
> > > > > > > 
> > > > > > > The patch doesn't say anything why it's useful. Could you tell why it's
> > > > > > > useful and inactive_list_is_low should be right place?
> > > > > > > 
> > > > > > > Don't get me wrong, please. I don't want to bother you.
> > > > > > > I really don't want to add random stuff although it's tracepoint for
> > > > > > > debugging.
> > > > > > 
> > > > > > This doesn't sounds random to me. We simply do not have a full picture
> > > > > > on 32b systems without this information. Especially when memcgs are
> > > > > > involved and global numbers spread over different LRUs.
> > > > > 
> > > > > Could you elaborate it?
> > > > 
> > > > The problem with 32b systems is that you only can consider a part of the
> > > > LRU for the lowmem requests. While we have global counters to see how
> > > > much lowmem inactive/active pages we have, those get distributed to
> > > > memcg LRUs. And that distribution is impossible to guess. So my thinking
> > > > is that it can become a real head scratcher to realize why certain
> > > > active LRUs are aged while others are not. This was the case when I was
> > > > debugging the last issue which triggered all this. All of the sudden I
> > > > have seen many invocations when inactive and active were zero which
> > > > sounded weird, until I realized that those are memcg's lruvec which is
> > > > what total numbers told me...
> > > 
> > > Hmm, it seems I miss something. AFAIU, what you need is just memcg
> > > identifier, not all lru size. If it isn't, please tell more detail
> > > usecase of all lru size in that particular tracepoint.
> > 
> > Having memcg id would be definitely helpful but that alone wouldn't tell
> > us how is the lowmem distributed. To be honest I really fail to see why
> > this bothers you all that much.
> 
> Because I fail to understand why you want to need additional all zone's
> LRU stat in inactive_list_is_low. With clear understanding, we can think
> over that it's really needed and right place to achieve the goal.
> 
> Could you say with a example you can think? It's really helpful to
> understand why it's needed.

OK, I feel I am repeating myself but let me try again. Without the
total_ numbers we really do not know how is the lowmem distributed over
lruvecs. There is simply no way to get this information from existing
counters if memcg is enabled.
 
> > [...]
> > > > > > I am not sure I am following. Why is the additional parameter a problem?
> > > > > 
> > > > > Well, to me, it's not a elegance. Is it? If we need such boolean variable
> > > > > to control show the trace, it means it's not a good place or think
> > > > > refactoring.
> > > > 
> > > > But, even when you refactor the code there will be other callers of
> > > > inactive_list_is_low outside of shrink_active_list...
> > > 
> > > Yes, that's why I said "it's okay if you love your version". However,
> > > we can do refactoring to remove "bool trace" and even, it makes code
> > > more readable, I believe.
> > > 
> > > >From 06eb7201d781155a8dee7e72fbb8423ec8175223 Mon Sep 17 00:00:00 2001
> > > From: Minchan Kim <minchan@...nel.org>
> > > Date: Fri, 13 Jan 2017 10:13:36 +0900
> > > Subject: [PATCH] mm: refactoring inactive_list_is_low
> > > 
> > > Recently, Michal Hocko added tracepoint into inactive_list_is_low
> > > for catching why VM decided to age the active list to know
> > > active/inacive balancing problem. With that, unfortunately, it
> > > added "bool trace" to inactlive_list_is_low to control some place
> > > should be prohibited tracing. It is not elegant to me so this patch
> > > try to clean it up.
> > > 
> > > Normally, most inactive_list_is_low is used for deciding active list
> > > demotion but one site(i.e., get_scan_count) uses for other purpose
> > > which reclaim file LRU forcefully. Sites for deactivation calls it
> > > with shrink_active_list. It means inactive_list_is_low could be
> > > located in shrink_active_list.
> > > 
> > > One more thing this patch does is to remove "ratio" in the tracepoint
> > > because we can get it by post processing in script via simple math.
> > > 
> > > Signed-off-by: Minchan Kim <minchan@...nel.org>
> > > ---
> > >  include/trace/events/vmscan.h |  9 +++-----
> > >  mm/vmscan.c                   | 51 ++++++++++++++++++++++++-------------------
> > >  2 files changed, 31 insertions(+), 29 deletions(-)
> > 
> > this cleanup adds more lines than it removes. I think reporting the
> 
> It's just marginal because the function names are really long and I want to
> keep a 80 column rule.
> Anyway, I'm not insisting on pushing this patch although I still think
> it's not nice to add "boolean variable" to control tracing or not.
> It's not a main interest.
> 
> > ratio is helpful because it doesn't cost us anything while calculating
> > it by later is just a bit annoying.
> 
> I really cannot imagine when inactive_ratio value is helpful for debugging.

without ratio you cannot really tell whether the inactive list is low.
As you say you can do that calculation in the trace data post processing
but why to bother with that if the number is that and we just need to
print it?

-- 
Michal Hocko
SUSE Labs

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