Re: [KVM-RFC PATCH 1/2] eventfd: add an explicit srcu based notifier interface

[Gregory Haskins <gregory.haskins@gmail.com>]

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[Adding Ingo]

Gregory Haskins wrote:
> Michael S. Tsirkin wrote:
>   
>> On Mon, Jun 15, 2009 at 10:29:56PM -0400, Gregory Haskins wrote:
>>   
>>     
>>> irqfd and its underlying implementation, eventfd, currently utilize
>>> the embedded wait-queue in eventfd for signal notification.  The nice thing
>>> about this design decision is that it re-uses the existing
>>> eventfd/wait-queue code and it generally works well....with several
>>> limitations.
>>>
>>> One of the limitations is that notification callbacks are always called
>>> inside a spin_lock_irqsave critical section.  Another limitation is
>>> that it is very difficult to build a system that can recieve release
>>> notification without being racy.
>>>
>>> Therefore, we introduce a new registration interface that is SRCU based
>>> instead of wait-queue based, and implement the internal wait-queue
>>> infrastructure in terms of this new interface.  We then convert irqfd
>>> to use this new interface instead of the existing wait-queue code.
>>>
>>> The end result is that we now have the opportunity to run the interrupt
>>> injection code serially to the callback (when the signal is raised from
>>> process-context, at least) instead of always deferring the injection to a
>>> work-queue.
>>>
>>> Signed-off-by: Gregory Haskins <ghaskins@novell.com>
>>> CC: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
>>> CC: Davide Libenzi <davidel@xmailserver.org>
>>> ---
>>>
>>>  fs/eventfd.c            |  115 +++++++++++++++++++++++++++++++++++++++++++----
>>>  include/linux/eventfd.h |   30 ++++++++++++
>>>  virt/kvm/eventfd.c      |  114 +++++++++++++++++++++--------------------------
>>>  3 files changed, 188 insertions(+), 71 deletions(-)
>>>
>>> diff --git a/fs/eventfd.c b/fs/eventfd.c
>>> index 72f5f8d..505d5de 100644
>>> --- a/fs/eventfd.c
>>> +++ b/fs/eventfd.c
>>> @@ -30,8 +30,47 @@ struct eventfd_ctx {
>>>  	 */
>>>  	__u64 count;
>>>  	unsigned int flags;
>>> +	struct srcu_struct srcu;
>>> +	struct list_head nh;
>>> +	struct eventfd_notifier notifier;
>>>  };
>>>  
>>> +static void _eventfd_wqh_notify(struct eventfd_notifier *en)
>>> +{
>>> +	struct eventfd_ctx *ctx = container_of(en,
>>> +					       struct eventfd_ctx,
>>> +					       notifier);
>>> +
>>> +	if (waitqueue_active(&ctx->wqh))
>>> +		wake_up_poll(&ctx->wqh, POLLIN);
>>> +}
>>> +
>>> +static void _eventfd_notify(struct eventfd_ctx *ctx)
>>> +{
>>> +	struct eventfd_notifier *en;
>>> +	int idx;
>>> +
>>> +	idx = srcu_read_lock(&ctx->srcu);
>>> +
>>> +	/*
>>> +	 * The goal here is to allow the notification to be preemptible
>>> +	 * as often as possible.  We cannot achieve this with the basic
>>> +	 * wqh mechanism because it requires the wqh->lock.  Therefore
>>> +	 * we have an internal srcu list mechanism of which the wqh is
>>> +	 * a client.
>>> +	 *
>>> +	 * Not all paths will invoke this function in process context.
>>> +	 * Callers should check for suitable state before assuming they
>>> +	 * can sleep (such as with preemptible()).  Paul McKenney assures
>>> +	 * me that srcu_read_lock is compatible with in-atomic, as long as
>>> +	 * the code within the critical section is also compatible.
>>> +	 */
>>> +	list_for_each_entry_rcu(en, &ctx->nh, list)
>>> +		en->ops->signal(en);
>>> +
>>> +	srcu_read_unlock(&ctx->srcu, idx);
>>> +}
>>> +
>>>  /*
>>>   * Adds "n" to the eventfd counter "count". Returns "n" in case of
>>>   * success, or a value lower then "n" in case of coutner overflow.
>>>     
>>>       
>> This is ugly, isn't it? With CONFIG_PREEMPT=no preemptible() is always false.
>>   
>>     
>
> As an aside, this is something I would like to address.  I keep running
> into this pattern where I could do something in-line if I had a
> "can_sleep()" context.  Otherwise, I have to punt to something like a
> workqueue which adds latency.  The closest thing I have to "can_sleep()"
> is preemptible(), which, as you correctly pointed out is limited to only
> working with CONFIG_PREEMPT=y.
>
> Its been a while since I looked into it, but one of the barriers that
> would need to be overcome is the fact that the preempt_count stuff gets
> compiled away with CONFIG_PREEMPT=n.  It is conceivable that we could
> make the preempt_count logic an independent config variable from
> CONFIG_PREEMPT to provide a can_sleep() macro without requiring
> full-blow preemption to be enabled.  So my questions would be as follows:
>
> a) Is the community conducive to such an idea?
> b) Are there other things to consider/fix besides the lack of
> preempt_count in order to implement such a beast?
>
> -Greg
>
>
>   
Hi Ingo,

Any thoughts here?

-Greg


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