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Date:   Fri, 24 Nov 2017 11:14:57 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Peter Enderborg <peter.enderborg@...y.com>
Cc:     linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        Jonathan Corbet <corbet@....net>,
        "Luis R . Rodriguez" <mcgrof@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Alex Deucher <alexander.deucher@....com>,
        "David S . Miller" <davem@...emloft.net>,
        Harry Wentland <Harry.Wentland@....com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Tony Cheng <Tony.Cheng@....com>,
        David Rientjes <rientjes@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jan Kara <jack@...e.cz>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Matthew Wilcox <willy@...ux.intel.com>,
        Hugh Dickins <hughd@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Kemi Wang <kemi.wang@...el.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        YASUAKI ISHIMATSU <yasu.isimatu@...il.com>,
        Nikolay Borisov <nborisov@...e.com>,
        Mel Gorman <mgorman@...e.de>,
        Pavel Tatashin <pasha.tatashin@...cle.com>
Subject: Re: [PATCH] mm:Add watermark slope for high mark

On Fri 24-11-17 11:07:07, Peter Enderborg wrote:
> When tuning the watermark_scale_factor to reduce stalls and compactions
> the high mark is also changed, it changed a bit too much. So this
> patch introduces a slope that can reduce this overhead a bit, or
> increase it if needed.

This doesn't explain what is the problem, why it is a problem and why we
need yet another tuning to address it. Users shouldn't really care about
internal stuff like watermark tuning for each watermark independently.
This looks like a gross hack. Please start over with the problem
description and then we can move on to an approapriate fix. Piling up
tuning knobs to workaround problems is simply not acceptable.
 
> Signed-off-by: Peter Enderborg <peter.enderborg@...y.com>
> ---
>  Documentation/sysctl/vm.txt | 15 +++++++++++++++
>  include/linux/mm.h          |  1 +
>  include/linux/mmzone.h      |  2 ++
>  kernel/sysctl.c             |  9 +++++++++
>  mm/page_alloc.c             |  6 +++++-
>  5 files changed, 32 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/sysctl/vm.txt b/Documentation/sysctl/vm.txt
> index eda628c..aecff6c 100644
> --- a/Documentation/sysctl/vm.txt
> +++ b/Documentation/sysctl/vm.txt
> @@ -62,6 +62,7 @@ Currently, these files are in /proc/sys/vm:
>  - user_reserve_kbytes
>  - vfs_cache_pressure
>  - watermark_scale_factor
> +- watermark_high_factor_slope
>  - zone_reclaim_mode
>  
>  ==============================================================
> @@ -857,6 +858,20 @@ that the number of free pages kswapd maintains for latency reasons is
>  too small for the allocation bursts occurring in the system. This knob
>  can then be used to tune kswapd aggressiveness accordingly.
>  
> +=============================================================
> +
> +watermark_high_factor_slope:
> +
> +This factor is high mark for watermark_scale_factor.
> +The unit is in percent.
> +Max value is 1000 and min value is 100. (High watermark is the same as
> +low water mark) Low watermark is min_wmark_pages + watermark_scale_factor.
> +and high watermark is
> +min_wmark_pages+(watermark_scale_factor * watermark_high_factor_slope).
> +
> +The default value is 200.
> +
> +
>  ==============================================================
>  
>  zone_reclaim_mode:
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 7661156..c89536b 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2094,6 +2094,7 @@ extern void zone_pcp_reset(struct zone *zone);
>  /* page_alloc.c */
>  extern int min_free_kbytes;
>  extern int watermark_scale_factor;
> +extern int watermark_high_factor_slope;
>  
>  /* nommu.c */
>  extern atomic_long_t mmap_pages_allocated;
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 67f2e3c..91bf842 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -886,6 +886,8 @@ int min_free_kbytes_sysctl_handler(struct ctl_table *, int,
>  					void __user *, size_t *, loff_t *);
>  int watermark_scale_factor_sysctl_handler(struct ctl_table *, int,
>  					void __user *, size_t *, loff_t *);
> +//int watermark_high_factor_tilt_sysctl_handler(struct ctl_table *, int,
> +//					void __user *, size_t *, loff_t *);
>  extern int sysctl_lowmem_reserve_ratio[MAX_NR_ZONES-1];
>  int lowmem_reserve_ratio_sysctl_handler(struct ctl_table *, int,
>  					void __user *, size_t *, loff_t *);
> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> index 2fb4e27..83c48c9 100644
> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -1444,6 +1444,15 @@ static struct ctl_table vm_table[] = {
>  		.extra2		= &one_thousand,
>  	},
>  	{
> +		.procname	= "watermark_high_factor_slope",
> +		.data		= &watermark_high_factor_slope,
> +		.maxlen		= sizeof(watermark_high_factor_slope),
> +		.mode		= 0644,
> +		.proc_handler	= watermark_scale_factor_sysctl_handler,
> +		.extra1		= &one_hundred,
> +		.extra2		= &one_thousand,
> +	},
> +	{
>  		.procname	= "percpu_pagelist_fraction",
>  		.data		= &percpu_pagelist_fraction,
>  		.maxlen		= sizeof(percpu_pagelist_fraction),
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 48b5b01..3dc50ff 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -263,6 +263,7 @@ compound_page_dtor * const compound_page_dtors[] = {
>  int min_free_kbytes = 1024;
>  int user_min_free_kbytes = -1;
>  int watermark_scale_factor = 10;
> +int watermark_high_factor_slope = 200;
>  
>  static unsigned long __meminitdata nr_kernel_pages;
>  static unsigned long __meminitdata nr_all_pages;
> @@ -6989,6 +6990,7 @@ static void __setup_per_zone_wmarks(void)
>  
>  	for_each_zone(zone) {
>  		u64 tmp;
> +		u64 tmp_high;
>  
>  		spin_lock_irqsave(&zone->lock, flags);
>  		tmp = (u64)pages_min * zone->managed_pages;
> @@ -7026,7 +7028,9 @@ static void __setup_per_zone_wmarks(void)
>  				      watermark_scale_factor, 10000));
>  
>  		zone->watermark[WMARK_LOW]  = min_wmark_pages(zone) + tmp;
> -		zone->watermark[WMARK_HIGH] = min_wmark_pages(zone) + tmp * 2;
> +		tmp_high = mult_frac(tmp, watermark_high_factor_slope, 100);
> +		zone->watermark[WMARK_HIGH] = min_wmark_pages(zone) + tmp_high;
> +
>  
>  		spin_unlock_irqrestore(&zone->lock, flags);
>  	}
> -- 
> 2.7.4
> 

-- 
Michal Hocko
SUSE Labs

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