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Date: Wed, 12 Feb 2020 10:25:55 +0800 From: Wei Yang <richardw.yang@...ux.intel.com> To: Mel Gorman <mgorman@...hsingularity.net> Cc: Wei Yang <richardw.yang@...ux.intel.com>, akpm@...ux-foundation.org, linux-mm@...ck.org, linux-kernel@...r.kernel.org, shakeelb@...gle.com, yang.shi@...ux.alibaba.com Subject: Re: [RFC Patch] mm/vmscan.c: not inherit classzone_idx from previous reclaim On Tue, Feb 11, 2020 at 10:42:23AM +0000, Mel Gorman wrote: >On Sun, Feb 09, 2020 at 03:41:45PM +0800, Wei Yang wrote: >> Before commit e716f2eb24de ("mm, vmscan: prevent kswapd sleeping >> prematurely due to mismatched classzone_idx"), classzone_idx could have >> two possibilities on a new loop based on whether there is a wakeup >> during reclaiming: >> >> * 0 if no wakeup >> * the classzone_idx request by wakeup >> >> As described in the changelog, this commit is willing to change the >> first case to (MAX_NR_ZONES - 1) to avoid some premature sleep. But it >> does not achieve the goal. >> >> There are two versions of kswapd_classzone_idx() since this change: >> >> * commit e716f2eb24de ("mm, vmscan: prevent kswapd sleeping >> prematurely due to mismatched classzone_idx") >> * commit dffcac2cb88e ("mm/vmscan.c: prevent useless kswapd loops") >> >> Both of them would return the classzone_idx we passed as the 2nd >> parameter when (pgdat->kswapd_classzone_idx == MAX_NR_ZONES). This >> means if there is no wakeup during reclaiming, we would use >> classzone_idx in previous round to sleep. >> > >This is somewhat intended. > >> This patch fixes the logic by using (MAX_NR_ZONES - 1) for the first >> case. >> > >Ok, what is the user-visible impact that is fixed by this patch or is >this based on code review only? Please describe the test case exactly >and the before and after results. I ask because this area is a magnet for >regressions and intuitive ideas often lead to counter-intuitive results. > This is based on code review only. I know your concern. This is an area more like art then engineering :-) Would you mind sharing some idea why we intend to inherit the classzone_idx? And for kswapd_order, we would restart at 0 if no wakeup during reclaim. I am curious about the idea behind this design :-) >-- >Mel Gorman >SUSE Labs -- Wei Yang Help you, Help me
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