From: Rik van Riel <riel@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>,
Frederic Weisbecker <fweisbec@gmail.com>
Cc: LKML <linux-kernel@vger.kernel.org>,
Dave Jones <davej@redhat.com>,
Thomas Gleixner <tglx@linutronix.de>,
Oleg Nesterov <oleg@redhat.com>,
"Paul E . McKenney" <paulmck@linux.vnet.ibm.com>,
Ingo Molnar <mingo@kernel.org>
Subject: Re: [PATCH 2/3] context_tracking: Inherit TIF_NOHZ through forks instead of context switches
Date: Thu, 02 Apr 2015 15:11:20 -0400 [thread overview]
Message-ID: <551D9458.9070502@redhat.com> (raw)
In-Reply-To: <20150402190910.GF27490@worktop.programming.kicks-ass.net>
On 04/02/2015 03:09 PM, Peter Zijlstra wrote:
> On Thu, Apr 02, 2015 at 07:39:24PM +0200, Frederic Weisbecker wrote:
>> TIF_NOHZ is used by context_tracking to force syscall slow-path on every
>> task in order to track userspace roundtrips. As such, it must be set on
>> all running tasks.
>>
>> It's currently explicitly inherited through context switches. There is
>> no need to do it on this fast-path though. The flag could be simply
>> set once for all on all tasks, whether they are running or not.
>>
>> Lets do this by setting the flag to init task on early boot and let it
>> propagate through fork inheritance.
>>
>
> One must ask, what's the point of the flag if everybody must always have
> it set?
We already test this word full of flags in the syscall
entry and exit path.
Testing this same word for an additional flag is cheaper
than testing a different variable.
See the places in entry_{32,64}.S where do_notify_resume,
syscall_trace_enter, syscall_trace_leave, etc get called.
All are called as a result of testing flags in the same
word.
next prev parent reply other threads:[~2015-04-02 19:11 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-04-02 17:39 [PATCH 0/3] context_tracking: A few improvements Frederic Weisbecker
2015-04-02 17:39 ` [PATCH 1/3] context_tracking: Protect against recursion Frederic Weisbecker
2015-04-02 18:01 ` Rik van Riel
2015-04-02 17:39 ` [PATCH 2/3] context_tracking: Inherit TIF_NOHZ through forks instead of context switches Frederic Weisbecker
2015-04-02 18:06 ` Rik van Riel
2015-04-02 18:08 ` Oleg Nesterov
2015-04-03 17:19 ` Frederic Weisbecker
2015-04-02 19:09 ` Peter Zijlstra
2015-04-02 19:11 ` Rik van Riel [this message]
2015-04-03 17:21 ` Frederic Weisbecker
2015-04-02 19:19 ` Oleg Nesterov
2015-04-02 17:39 ` [PATCH 3/3] context_tracking: Tag init code Frederic Weisbecker
2015-04-02 18:07 ` Rik van Riel
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