From: Minchan Kim Subject: Re: [PATCH 4/4] mm: vmscan: If kswapd has been running too long, allow it to sleep Date: Mon, 16 May 2011 14:04:00 +0900 Message-ID: References: <1305295404-12129-1-git-send-email-mgorman@suse.de> <1305295404-12129-5-git-send-email-mgorman@suse.de> <4DCFAA80.7040109@jp.fujitsu.com> <1305519711.4806.7.camel@mulgrave.site> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable Cc: KOSAKI Motohiro , mgorman@suse.de, akpm@linux-foundation.org, colin.king@canonical.com, raghu.prabhu13@gmail.com, jack@suse.cz, chris.mason@oracle.com, cl@linux.com, penberg@kernel.org, riel@redhat.com, hannes@cmpxchg.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-ext4@vger.kernel.org To: James Bottomley Return-path: In-Reply-To: <1305519711.4806.7.camel@mulgrave.site> Sender: owner-linux-mm@kvack.org List-Id: linux-ext4.vger.kernel.org On Mon, May 16, 2011 at 1:21 PM, James Bottomley wrote: > On Sun, 2011-05-15 at 19:27 +0900, KOSAKI Motohiro wrote: >> (2011/05/13 23:03), Mel Gorman wrote: >> > Under constant allocation pressure, kswapd can be in the situation whe= re >> > sleeping_prematurely() will always return true even if kswapd has been >> > running a long time. Check if kswapd needs to be scheduled. >> > >> > Signed-off-by: Mel Gorman >> > --- >> > =C2=A0 mm/vmscan.c | =C2=A0 =C2=A04 ++++ >> > =C2=A0 1 files changed, 4 insertions(+), 0 deletions(-) >> > >> > diff --git a/mm/vmscan.c b/mm/vmscan.c >> > index af24d1e..4d24828 100644 >> > --- a/mm/vmscan.c >> > +++ b/mm/vmscan.c >> > @@ -2251,6 +2251,10 @@ static bool sleeping_prematurely(pg_data_t *pgd= at, int order, long remaining, >> > =C2=A0 =C2=A0 unsigned long balanced =3D 0; >> > =C2=A0 =C2=A0 bool all_zones_ok =3D true; >> > >> > + =C2=A0 /* If kswapd has been running too long, just sleep */ >> > + =C2=A0 if (need_resched()) >> > + =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 return false; >> > + >> >> Hmm... I don't like this patch so much. because this code does >> >> - don't sleep if kswapd got context switch at shrink_inactive_list > > This isn't entirely true: =C2=A0need_resched() will be false, so we'll fo= llow > the normal path for determining whether to sleep or not, in effect > leaving the current behaviour unchanged. > >> - sleep if kswapd didn't > > This also isn't entirely true: whether need_resched() is true at this > point depends on a whole lot more that whether we did a context switch > in shrink_inactive. It mostly depends on how long we've been running > without giving up the CPU. =C2=A0Generally that will mean we've been roun= d > the shrinker loop hundreds to thousands of times without sleeping. > >> It seems to be semi random behavior. > > Well, we have to do something. =C2=A0Chris Mason first suspected the hang= was > a kswapd rescheduling problem a while ago. =C2=A0We tried putting > cond_rescheds() in several places in the vmscan code, but to no avail. Is it a result of test with patch of Hannes(ie, !pgdat_balanced)? If it isn't, it would be nop regardless of putting cond_reshed at vmscan.c. Because, although we complete zone balancing, kswapd doesn't sleep as pgdat_balance returns wrong result. And at last VM calls balance_pgdat. In this case, balance_pgdat returns without any work as kswap couldn't find zones which have not enough free pages and goto out. kswapd could repeat this work infinitely. So you don't have a chance to call cond_resched. But if your test was with Hanne's patch, I am very curious how come kswapd consumes CPU a lot. > The need_resched() in sleeping_prematurely() seems to be about the best > option. =C2=A0The other option might be just to put a cond_resched() in > kswapd_try_to_sleep(), but that will really have about the same effect. I don't oppose it but before that, I think we have to know why kswapd consumes CPU a lot although we applied Hannes' patch. > > James > > > -- > To unsubscribe, send a message with 'unsubscribe linux-mm' in > the body to majordomo@kvack.org. =C2=A0For more info on Linux MM, > see: http://www.linux-mm.org/ . > Fight unfair telecom internet charges in Canada: sign http://stopthemeter= .ca/ > Don't email: email@kvack.org > --=20 Kind regards, Minchan Kim -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: email@kvack.org