From: Mike Kravetz <mike.kravetz@oracle.com>
To: David Rientjes <rientjes@google.com>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org,
Song Liu <songliubraving@fb.com>,
"Kirill A.Shutemov" <kirill.shutemov@linux.intel.com>,
Mel Gorman <mgorman@techsingularity.net>,
Vlastimil Babka <vbabka@suse.cz>,
Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH] mm: always consider THP when adjusting min_free_kbytes
Date: Tue, 4 Feb 2020 13:42:43 -0800 [thread overview]
Message-ID: <8cc18928-0b52-7c2e-fbc6-5952eb9b06ab@oracle.com> (raw)
In-Reply-To: <alpine.DEB.2.21.2002041218580.58724@chino.kir.corp.google.com>
On 2/4/20 12:33 PM, David Rientjes wrote:
> On Tue, 4 Feb 2020, Mike Kravetz wrote:
>
> Hmm, if khugepaged_adjust_min_free_kbytes() increases min_free_kbytes for
> thp, then the user has no ability to override this increase by using
> vm.min_free_kbytes?
>
> IIUC, with this change, it looks like memory hotplug events properly
> increase min_free_kbytes for thp optimization but also doesn't respect a
> previous user-defined value?
Good catch.
We should only call khugepaged_adjust_min_free_kbytes from the 'true'
block of this if statement in init_per_zone_wmark_min.
if (new_min_free_kbytes > user_min_free_kbytes) {
min_free_kbytes = new_min_free_kbytes;
if (min_free_kbytes < 128)
min_free_kbytes = 128;
if (min_free_kbytes > 65536)
min_free_kbytes = 65536;
} else {
pr_warn("min_free_kbytes is not updated to %d because user defined value %d is preferred\n",
new_min_free_kbytes, user_min_free_kbytes);
}
In the existing code, a hotplug event will cause min_free_kbytes to overwrite
the user defined value if the new value is greater. However, you will get
the warning message if the user defined value is greater. I am not sure if
this is the 'desired/expected' behavior? We print a warning if the user value
takes precedence over our calculated value. However, we do not print a message
if we overwrite the user defined value. That doesn't seem right!
> So it looks like this is fixing an obvious correctness issue but also now
> requires users to rewrite the sysctl if they want to decrease the min
> watermark.
Moving the call to khugepaged_adjust_min_free_kbytes as described above
would avoid the THP adjustment unless we were going to overwrite the
user defined value. Now, I am not sure overwriting the user defined value
as is done today is actually the correct thing to do.
Thoughts?
Perhaps we should never overwrite a user defined value?
--
Mike Kravetz
next prev parent reply other threads:[~2020-02-04 21:44 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-02-04 19:41 [PATCH] mm: always consider THP when adjusting min_free_kbytes Mike Kravetz
2020-02-04 20:33 ` David Rientjes
2020-02-04 21:42 ` Mike Kravetz [this message]
2020-02-04 21:53 ` Matthew Wilcox
2020-02-05 0:33 ` Mike Kravetz
2020-02-06 1:36 ` Mike Kravetz
2020-02-06 20:09 ` Khalid Aziz
2020-02-06 20:39 ` Matthew Wilcox
2020-02-06 21:23 ` Mike Kravetz
2020-02-06 21:32 ` Matthew Wilcox
2020-02-10 18:58 ` Mike Kravetz
2020-02-04 23:37 ` Khalid Aziz
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