From: Kirill Tkhai <email@example.com> To: Cyrill Gorcunov <firstname.lastname@example.org> Cc: email@example.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, email@example.com Subject: Re: [PATCH v2] ksm: React on changing "sleep_millisecs" parameter faster Date: Tue, 11 Dec 2018 17:15:00 +0300 Message-ID: <firstname.lastname@example.org> (raw) In-Reply-To: <20181211123455.GI2342@uranus.lan> On 11.12.2018 15:34, Cyrill Gorcunov wrote: > On Tue, Dec 11, 2018 at 03:22:42PM +0300, Kirill Tkhai wrote: >> On 11.12.2018 14:13, Cyrill Gorcunov wrote: >>> On Tue, Dec 11, 2018 at 01:26:59PM +0300, Kirill Tkhai wrote: >>>> ksm thread unconditionally sleeps in ksm_scan_thread() >>>> after each iteration: >>>> >>>> schedule_timeout_interruptible( >>>> msecs_to_jiffies(ksm_thread_sleep_millisecs)) >>>> >>>> The timeout is configured in /sys/kernel/mm/ksm/sleep_millisecs. >>>> >>>> In case of user writes a big value by a mistake, and the thread >>>> enters into schedule_timeout_interruptible(), it's not possible >>>> to cancel the sleep by writing a new smaler value; the thread >>>> is just sleeping till timeout expires. >>>> >>>> The patch fixes the problem by waking the thread each time >>>> after the value is updated. >>>> >>>> This also may be useful for debug purposes; and also for userspace >>>> daemons, which change sleep_millisecs value in dependence of >>>> system load. >>>> >>>> Signed-off-by: Kirill Tkhai <email@example.com> >>>> >>>> v2: Use wait_event_interruptible_timeout() instead of unconditional >>>> schedule_timeout(). >>> ... >>>> @@ -2844,7 +2849,10 @@ static ssize_t sleep_millisecs_store(struct kobject *kobj, >>>> if (err || msecs > UINT_MAX) >>>> return -EINVAL; >>>> >>>> + mutex_lock(&ksm_thread_mutex); >>>> ksm_thread_sleep_millisecs = msecs; >>>> + mutex_unlock(&ksm_thread_mutex); >>>> + wake_up_interruptible(&ksm_iter_wait); >>> >>> Btw, just thought -- if we start using this mutex here don't we >>> open a window for force attack on the thread self execution, >>> iow if there gonna be a million of writers do we have a guarantee >>> thread ksm_scan_thread will grab the mutex earlier than writers >>> (or somewhere inbetween)? >> >> This file is permitted for global root only. I don't think there is >> a problem. >> >> If someone wants to make ksm helpless, a person may just write a big >> "sleep_millisecs" value. KSM thread won't be executed almost all the time >> in this case. > > True. Still I think if we can leave without taking a lock it a rule of thumb. > Something like > > if (msecs != ksm_thread_sleep_millisecs) > wake_up_interruptable(&ksm_iter_wait); > > Thoughts? Ok, good idea.
prev parent reply index Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top 2018-12-11 10:03 [PATCH] " Cyrill Gorcunov 2018-12-11 10:26 ` [PATCH v2] " Kirill Tkhai 2018-12-11 11:13 ` Cyrill Gorcunov 2018-12-11 12:22 ` Kirill Tkhai 2018-12-11 12:34 ` Cyrill Gorcunov 2018-12-11 14:15 ` Kirill Tkhai [this message]
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