Re: [PATCH v2] epoll: Support for disabling items, and a self-testapp.

From: Paul Holland
Date: Fri Oct 19 2012 - 09:39:59 EST

On 10/19/12 6:03 AM, "Paolo Bonzini" <pbonzini@xxxxxxxxxx> wrote:

>Il 18/10/2012 20:05, Andy Lutomirski ha scritto:
>> Unless something is rather buggy in kernel land (and I don't think it
>> is), once EPOLL_CTL_DEL has returned, no call to epoll_wait that starts
>> *after* EPOLL_CTL_DEL finishes will return that object. This suggests
>> an RCU-like approach: once EPOLL_CTL_DEL has returned and every thread
>> has returned from an epoll_wait call that started after the
>> EPOLL_CTL_DEL returns, then the data structure can be safely freed.
>> In pseudocode:
>> delete(fd, pdata) {
>> pdata->dead = true;
>> rcu_call(delete pdata);
>> }
>> wait() {
>> epoll_wait;
>> for each event pdata {
>> if (pdata->gone) continue;
>> process the event;
>> }
>> rcu_this_is_a_grace_period();
>> }
>> Of course, these are not normal grace periods and would need to be
>> tracked separately. (The optimal data structure to do this without
>> killing scalability is not obvious. urcu presumably implements such a
>> thing.)
>> Am I right?
>Equip each thread with a) an id or something else that lets each thread
>refer to "the next" thread; b) a lists of "items waiting to be deleted".
> Then the deleting thread adds the item being deleted to the first
>thread's list. Before executing epoll_wait, thread K empties its list
>and passes the buck, appending the old contents of its list to that of
>thread K+1. This is an O(1) operation no matter how many items are
>being deleted; only Thread N, being the last thread, actually has to go
>through the list and delete the items.
>The lists need to be protected by a mutex, but contention should really
>be rare since there are just two writers. Note that each thread only
>needs to hold one mutex at a time, and the deletion loop does not need
>to happen with the mutex held at all, so there's no worries of
>"cascading" waits on the mutexes.
>Compared to, you get
>rid of the per-item mutex and the operations that have to be done with
>the (now per-thread) mutex held remain pretty trivial.

A disadvantage of solutions in this direction, which was not preset in
Paton's patch, is that all calls to epoll_wait would need to specify some
timeout value (!= -1) to guarantee that they each come out of epoll_wait
and execute the "pass the buck" or "grace_period" logic. So you would
then have contention between designs that want highly responsive "delete"
operations (those would require very short timeout values to epoll_wait)
and those that want low execution overhead (those would want larger
timeout values).


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