| Message ID | 20210201060651.3781-1-osalvador@suse.de (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | mm: workingset: clarify eviction order and distance calculation | expand |
On 2/1/21 7:06 AM, Oscar Salvador wrote: > The premise of the refault distance is that it can be seen as a deficit > of the inactive list space, so that if the inactive list would have had > (R - E) more slots, the page would not have been evicted but promoted > to the active list instead. > > However, the way the code is ordered right now set us to be off by one, > so the real number of slots would be (R - E) + 1. > I stumbled upon this when trying to understand the code and it puzzled me > that the comments did not match what the code did. > > This it not an issue at all since evictions and refaults tend to happen > in a number large enough that being off-by-one does not have any impact > - and since the compiler and CPUs are free to rearrange the execution > sequence anyway. > But as Johannes says, it is better to re-arrange the code in the proper > order since otherwise would be misleading to somebody who is actively > reading and trying to understand the logic of the code - like it > happened to me. > > Signed-off-by: Oscar Salvador <osalvador@suse.de> > Acked-by: Johannes Weiner <hannes@cmpxchg.org> Acked-by: Vlastimil Babka <vbabka@suse.cz> > --- > mm/workingset.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/mm/workingset.c b/mm/workingset.c > index 10e96de945b3..0201aa1ff320 100644 > --- a/mm/workingset.c > +++ b/mm/workingset.c > @@ -263,10 +263,10 @@ void *workingset_eviction(struct page *page, struct mem_cgroup *target_memcg) > VM_BUG_ON_PAGE(!PageLocked(page), page); > > lruvec = mem_cgroup_lruvec(target_memcg, pgdat); > - workingset_age_nonresident(lruvec, thp_nr_pages(page)); > /* XXX: target_memcg can be NULL, go through lruvec */ > memcgid = mem_cgroup_id(lruvec_memcg(lruvec)); > eviction = atomic_long_read(&lruvec->nonresident_age); > + workingset_age_nonresident(lruvec, thp_nr_pages(page)); > return pack_shadow(memcgid, pgdat, eviction, PageWorkingset(page)); > } > >
On Mon, Feb 01, 2021 at 07:06:51AM +0100, Oscar Salvador wrote: > The premise of the refault distance is that it can be seen as a deficit > of the inactive list space, so that if the inactive list would have had > (R - E) more slots, the page would not have been evicted but promoted > to the active list instead. > > However, the way the code is ordered right now set us to be off by one, > so the real number of slots would be (R - E) + 1. > I stumbled upon this when trying to understand the code and it puzzled me > that the comments did not match what the code did. > > This it not an issue at all since evictions and refaults tend to happen > in a number large enough that being off-by-one does not have any impact > - and since the compiler and CPUs are free to rearrange the execution > sequence anyway. > But as Johannes says, it is better to re-arrange the code in the proper > order since otherwise would be misleading to somebody who is actively > reading and trying to understand the logic of the code - like it > happened to me. > > Signed-off-by: Oscar Salvador <osalvador@suse.de> > Acked-by: Johannes Weiner <hannes@cmpxchg.org> Hi Andrew, is this on your radar? Thanks!
On Thu, Feb 11, 2021 at 10:26:45PM +0100, Oscar Salvador wrote: > Hi Andrew, > > is this on your radar? Please, disregard this, I was obviously blind as I did not spot it in mmotm. > > Thanks! > > -- > Oscar Salvador > SUSE L3 >
diff --git a/mm/workingset.c b/mm/workingset.c index 10e96de945b3..0201aa1ff320 100644 --- a/mm/workingset.c +++ b/mm/workingset.c @@ -263,10 +263,10 @@ void *workingset_eviction(struct page *page, struct mem_cgroup *target_memcg) VM_BUG_ON_PAGE(!PageLocked(page), page); lruvec = mem_cgroup_lruvec(target_memcg, pgdat); - workingset_age_nonresident(lruvec, thp_nr_pages(page)); /* XXX: target_memcg can be NULL, go through lruvec */ memcgid = mem_cgroup_id(lruvec_memcg(lruvec)); eviction = atomic_long_read(&lruvec->nonresident_age); + workingset_age_nonresident(lruvec, thp_nr_pages(page)); return pack_shadow(memcgid, pgdat, eviction, PageWorkingset(page)); }