Messages in this thread | | | From | (Eric W. Biederman) | Subject | Re: [PATCH] move put_task_struct() reaping into a thread [Re: 2.6.18-rt1] | Date | Wed, 27 Sep 2006 00:02:21 -0600 |
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Bill Huey (hui) <billh@gnuppy.monkey.org> writes:
> On Tue, Sep 26, 2006 at 08:55:41PM -0600, Eric W. Biederman wrote: >> Bill Huey (hui) <billh@gnuppy.monkey.org> writes: >> > This patch moves put_task_struct() reaping into a thread instead of an >> > RCU callback function as discussed with Esben publically and Ingo privately: >> >> Stupid question. > > It's a great question actually. > >> Why does the rt tree make all calls to put_task_struct an rcu action? >> We only need the rcu callback from kernel/exit.c > > Because the conversion of memory allocation routines like kmalloc and kfree aren't > safely callable within a preempt_disable critical section since they were incompletely > converted in the -rt. It can run into the sleeping in atomic scenario which can result > in a deadlock since those routines use blocking locks internally in the implementation > now as a result of the spinlock_t conversion to blocking locks.
Interesting. I think the easy solution would just be to assert that put_task_struct can sleep and to fix any callers that expect differently. I haven't looked very closely but I don't recall anything that needs put_task_struct to be atomic. With a function that complex I certainly would not expect it to never sleep unless it had a big fat comment.
Well I did find an instance where we call put_task_struct with a spinlock held. Inside of lib/rwsem.c:rwsem_down_failed_common().
Still that may be the only user that cares. I suspect with a little code rearrangement that case is fixable. It's not like that code is a fast path or anything. It should just be a matter of passing the task struct outside of the spinlock before calling put_task_struct.
>> Nothing else needs those semantics. > > Right, blame it on the incomplete conversion of the kmalloc and friends. GFP_ATOMIC is > is kind of meaningless in the -rt tree and it might be a good thing to add something > like GFP_RT_ATOMIC for cases like this to be handled properly and restore that > particular semantic in a more meaningful way.
But this is a path where we are freeing data, so GFP_ATOMIC should not come into it. I just read through the code and there are not allocations there.
>> I agree that put_task_struct is the most common point so this is unlikely >> to remove your issues with rcu callbacks but it just seems completely backwards >> to increase the number of rcu callbacks in the rt tree. > > I'm not sure what mean here, but if you mean that you don't like the RCU API abuse then > I agree with you on that. However, Ingo disagrees and I'm not going to argue it with him. > Although, I'm not going stop you if you do. :)
What I was thinking is that rcu isn't terribly friendly to realtime activities because it postpones work and can wind up with a lot of work to do at some random time later which can be bad for latencies.
So I was very surprised to see an rt patch making more things under rcu protection. Especially as I have heard other developers worried about rt issues discussing removing the rcu functionality.
My gut feel now that I understand the pieces is that this approach has all of the hallmarks of a hack, both the kmalloc/kfree thing and even more calling put_task_struct in an atomic context. If the callers were fixed put_task_struct could safely sleep so kmalloc/kfree sleeping would not be a problem.
Eric
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