2017-11-28 18:25 GMT+08:00 Jan Kara <jack@xxxxxxx>:
> Hi Yafang,
>
> On Tue 28-11-17 11:11:40, Yafang Shao wrote:
>> What about bellow change ?
>> It makes the function domain_dirty_limits() more clear.
>> And the result will have a higher precision.
>
> Frankly, I don't find this any better and you've just lost the additional
> precision of ratios computed in the "if (gdtc)" branch the multiplication by
> PAGE_SIZE got us.
>
What about bellow change? It won't be lost any more, becasue
bytes and bg_bytes are both PAGE_SIZE aligned.
- if (bytes)
- ratio = min(DIV_ROUND_UP(bytes, global_avail),
- PAGE_SIZE);
- if (bg_bytes)
- bg_ratio = min(DIV_ROUND_UP(bg_bytes, global_avail),
- PAGE_SIZE);
+ if (bytes) {
+ pages = DIV_ROUND_UP(bytes, PAGE_SIZE);
+ ratio = DIV_ROUND_UP(pages * 100, global_avail);
+
+ }
+
+ if (bg_bytes) {
+ pages = DIV_ROUND_UP(bg_bytes, PAGE_SIZE);
+ bg_ratio = DIV_ROUND_UP(pages * 100, global_avail);
+ }
>
>
>> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
>> index 8a15511..2b5e507 100644
>> --- a/mm/page-writeback.c
>> +++ b/mm/page-writeback.c
>> @@ -397,8 +397,8 @@ static void domain_dirty_limits(struct
>> dirty_throttle_control *dtc)
>> unsigned long bytes = vm_dirty_bytes;
>> unsigned long bg_bytes = dirty_background_bytes;
>> /* convert ratios to per-PAGE_SIZE for higher precision */
>> - unsigned long ratio = (vm_dirty_ratio * PAGE_SIZE) / 100;
>> - unsigned long bg_ratio = (dirty_background_ratio * PAGE_SIZE) / 100;
>> + unsigned long ratio = vm_dirty_ratio;
>> + unsigned long bg_ratio = dirty_background_ratio;
>> unsigned long thresh;
>> unsigned long bg_thresh;
>> struct task_struct *tsk;
>> @@ -416,28 +416,33 @@ static void domain_dirty_limits(struct
>> dirty_throttle_control *dtc)
>> */
>> if (bytes)
>> ratio = min(DIV_ROUND_UP(bytes, global_avail),
>> - PAGE_SIZE);
>> + 100);
>> if (bg_bytes)
>> bg_ratio = min(DIV_ROUND_UP(bg_bytes, global_avail),
>> - PAGE_SIZE);
>> + 99); /* bg_ratio should less than ratio */
>> bytes = bg_bytes = 0;
>> }
>>
>> + /* bytes and bg_bytes must be PAGE_SIZE aligned */
>> if (bytes)
>> - thresh = DIV_ROUND_UP(bytes, PAGE_SIZE);
>> + thresh = DIV_ROUND_UP(bytes, PAGE_SIZE) * 100;
>> else
>> - thresh = (ratio * available_memory) / PAGE_SIZE;
>> + thresh = ratio * available_memory;
>>
>> if (bg_bytes)
>> - bg_thresh = DIV_ROUND_UP(bg_bytes, PAGE_SIZE);
>> + bg_thresh = DIV_ROUND_UP(bg_bytes, PAGE_SIZE) * 100;
>> else
>> - bg_thresh = (bg_ratio * available_memory) / PAGE_SIZE;
>> + bg_thresh = bg_ratio * available_memory;
>>
>> if (unlikely(bg_thresh >= thresh)) {
>> pr_warn("vm direct limit must be set greater than background limit.\n");
>> bg_thresh = thresh / 2;
>> }
>>
>> + /* ensure bg_thresh and thresh never be 0 */
>> + bg_thresh = DIV_ROUND_UP(bg_thresh, 100);
>> + thresh = DIV_ROUND_UP(thresh, 100);
>> +
>> tsk = current;
>> if (tsk->flags & PF_LESS_THROTTLE || rt_task(tsk)) {
>>
>> 2017-11-27 17:19 GMT+08:00 Michal Hocko <mhocko@xxxxxxxx>:
>> > Andrew,
>> > could you simply send this to Linus. If we _really_ need something to
>> > prevent misconfiguration, which I doubt to be honest, then it should be
>> > thought through much better.
>> > ---
>> > From 4ef6b1cbf98ea5dae155ab3303c4ae1d93411b79 Mon Sep 17 00:00:00 2001
>> > From: Michal Hocko <mhocko@xxxxxxxx>
>> > Date: Mon, 27 Nov 2017 10:12:15 +0100
>> > Subject: [PATCH] Revert "mm/page-writeback.c: print a warning if the vm
>> > dirtiness settings are illogical"
>> >
>> > This reverts commit 0f6d24f878568fac579a1962d0bf7cb9f01e0ceb because
>> > it causes false positive warnings during OOM situations as noticed by
>> > Tetsuo Handa:
>> > [ 621.814512] Node 0 active_anon:3525940kB inactive_anon:8372kB active_file:216kB inactive_file:1872kB unevictable:0kB isolated(anon):0kB isolated(file):0kB mapped:2504kB dirty:52kB writeback:0kB shmem:8660kB s
>> > hmem_thp: 0kB shmem_pmdmapped: 0kB anon_thp: 636928kB writeback_tmp:0kB unstable:0kB all_unreclaimable? yes
>> > [ 621.821534] Node 0 DMA free:14848kB min:284kB low:352kB high:420kB active_anon:992kB inactive_anon:0kB active_file:0kB inactive_file:0kB unevictable:0kB writepending:0kB present:15988kB managed:15904kB mlocke
>> > d:0kB kernel_stack:0kB pagetables:24kB bounce:0kB free_pcp:0kB local_pcp:0kB free_cma:0kB
>> > [ 621.829035] lowmem_reserve[]: 0 2687 3645 3645
>> > [ 621.831655] Node 0 DMA32 free:53004kB min:49608kB low:62008kB high:74408kB active_anon:2712648kB inactive_anon:0kB active_file:0kB inactive_file:0kB unevictable:0kB writepending:0kB present:3129216kB managed:
>> > 2773132kB mlocked:0kB kernel_stack:96kB pagetables:5096kB bounce:0kB free_pcp:0kB local_pcp:0kB free_cma:0kB
>> > [ 621.839945] lowmem_reserve[]: 0 0 958 958
>> > [ 621.842811] Node 0 Normal free:17140kB min:17684kB low:22104kB high:26524kB active_anon:812300kB inactive_anon:8372kB active_file:1228kB inactive_file:1868kB unevictable:0kB writepending:52kB present:1048576k
>> > B managed:981224kB mlocked:0kB kernel_stack:3520kB pagetables:8552kB bounce:0kB free_pcp:120kB local_pcp:120kB free_cma:0kB
>> > [ 621.852473] lowmem_reserve[]: 0 0 0 0
>> > [...]
>> > [ 621.891477] Out of memory: Kill process 8459 (a.out) score 999 or sacrifice child
>> > [ 621.894363] Killed process 8459 (a.out) total-vm:4180kB, anon-rss:88kB, file-rss:0kB, shmem-rss:0kB
>> > [ 621.897172] oom_reaper: reaped process 8459 (a.out), now anon-rss:0kB, file-rss:0kB, shmem-rss:0kB
>> > [ 622.424664] vm direct limit must be set greater than background limit.
>> >
>> > The problem is that both thresh and bg_thresh will be 0 if available_memory
>> > is less than 4 pages when evaluating global_dirtyable_memory. While
>> > this might be worked around the whole point of the warning is dubious at
>> > best. We do rely on admins to do sensible things when changing tunable
>> > knobs. Dirty memory writeback knobs are not any special in that regards
>> > so revert the warning rather than adding more hacks to work this around.
>> >
>> > Rerported-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
>> > Debugged-by: Yafang Shao <laoar.shao@xxxxxxxxx>
>> > Fixes: 0f6d24f87856 ("mm/page-writeback.c: print a warning if the vm dirtiness settings are illogical")
>> > Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
>> > ---
>> > Documentation/sysctl/vm.txt | 7 -------
>> > mm/page-writeback.c | 5 +----
>> > 2 files changed, 1 insertion(+), 11 deletions(-)
>> >
>> > diff --git a/Documentation/sysctl/vm.txt b/Documentation/sysctl/vm.txt
>> > index b920423f88cb..5025ff9307e6 100644
>> > --- a/Documentation/sysctl/vm.txt
>> > +++ b/Documentation/sysctl/vm.txt
>> > @@ -158,10 +158,6 @@ Note: the minimum value allowed for dirty_bytes is two pages (in bytes); any
>> > value lower than this limit will be ignored and the old configuration will be
>> > retained.
>> >
>> > -Note: the value of dirty_bytes also must be set greater than
>> > -dirty_background_bytes or the amount of memory corresponding to
>> > -dirty_background_ratio.
>> > -
>> > ==============================================================
>> >
>> > dirty_expire_centisecs
>> > @@ -181,9 +177,6 @@ generating disk writes will itself start writing out dirty data.
>> >
>> > The total available memory is not equal to total system memory.
>> >
>> > -Note: dirty_ratio must be set greater than dirty_background_ratio or
>> > -ratio corresponding to dirty_background_bytes.
>> > -
>> > ==============================================================
>> >
>> > dirty_writeback_centisecs
>> > diff --git a/mm/page-writeback.c b/mm/page-writeback.c
>> > index e7095030aa1f..586f31261c83 100644
>> > --- a/mm/page-writeback.c
>> > +++ b/mm/page-writeback.c
>> > @@ -433,11 +433,8 @@ static void domain_dirty_limits(struct dirty_throttle_control *dtc)
>> > else
>> > bg_thresh = (bg_ratio * available_memory) / PAGE_SIZE;
>> >
>> > - if (unlikely(bg_thresh >= thresh)) {
>> > - pr_warn("vm direct limit must be set greater than background limit.\n");
>> > + if (bg_thresh >= thresh)
>> > bg_thresh = thresh / 2;
>> > - }
>> > -
>> > tsk = current;
>> > if (tsk->flags & PF_LESS_THROTTLE || rt_task(tsk)) {
>> > bg_thresh += bg_thresh / 4 + global_wb_domain.dirty_limit / 32;
>> > --
>> > 2.15.0
>> >
>> > --
>> > Michal Hocko
>> > SUSE Labs
> --
> Jan Kara <jack@xxxxxxxx>
> SUSE Labs, CR
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