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SubjectRe: [PATCH] ipc/msg.c: wake up senders until there is a queue empty capacity
Hello, Manfred!

Thank you, for your review. I've reviewed your patch.

I forgot about the case with different message types. At now with your patch,
a sender will force message consuming if that doesn't hold own capacity.

I have measured queue throughput and have pushed the results to:
https://github.com/artur-barsegyan/systemv_queue_research

But I'm confused about the next thought: in general loop in the do_msgsnd()
function, we doesn't check pipeline sending availability. Your case will be
optimized if we check the pipeline sending inside the loop.

On Sun, May 24, 2020 at 03:21:31PM +0200, Manfred Spraul wrote:
> Hello Artur,
>
> On 5/23/20 10:34 PM, Artur Barsegyan wrote:
> > Take into account the total size of the already enqueued messages of
> > previously handled senders before another one.
> >
> > Otherwise, we have serious degradation of receiver throughput for
> > case with multiple senders because another sender wakes up,
> > checks the queue capacity and falls into sleep again.
> >
> > Each round-trip wastes CPU time a lot and leads to perceptible
> > throughput degradation.
> >
> > Source code of:
> > - sender/receiver
> > - benchmark script
> > - ready graphics of before/after results
> >
> > is located here: https://github.com/artur-barsegyan/systemv_queue_research
>
> Thanks for analyzing the issue!
>
> > Signed-off-by: Artur Barsegyan <a.barsegyan96@gmail.com>
> > ---
> > ipc/msg.c | 4 +++-
> > 1 file changed, 3 insertions(+), 1 deletion(-)
> >
> > diff --git a/ipc/msg.c b/ipc/msg.c
> > index caca67368cb5..52d634b0a65a 100644
> > --- a/ipc/msg.c
> > +++ b/ipc/msg.c
> > @@ -214,6 +214,7 @@ static void ss_wakeup(struct msg_queue *msq,
> > struct msg_sender *mss, *t;
> > struct task_struct *stop_tsk = NULL;
> > struct list_head *h = &msq->q_senders;
> > + size_t msq_quota_used = 0;
> > list_for_each_entry_safe(mss, t, h, list) {
> > if (kill)
> > @@ -233,7 +234,7 @@ static void ss_wakeup(struct msg_queue *msq,
> > * move the sender to the tail on behalf of the
> > * blocked task.
> > */
> > - else if (!msg_fits_inqueue(msq, mss->msgsz)) {
> > + else if (!msg_fits_inqueue(msq, msq_quota_used + mss->msgsz)) {
> > if (!stop_tsk)
> > stop_tsk = mss->tsk;
> > @@ -241,6 +242,7 @@ static void ss_wakeup(struct msg_queue *msq,
> > continue;
> > }
> > + msq_quota_used += mss->msgsz;
> > wake_q_add(wake_q, mss->tsk);
>
> You have missed the case of a do_msgsnd() that doesn't enqueue the message:
>
> Situation:
>
> - 2 messages of type 1 in the queue (2x8192 bytes, queue full)
>
> - 6 senders waiting to send messages of type 2
>
> - 6 receivers waiting to get messages of type 2.
>
> If now a receiver reads one message of type 1, then all 6 senders can send.
>
> WIth your patch applied, only one sender sends the message to one receiver,
> and the remaining 10 tasks continue to sleep.
>
>
> Could you please check and (assuming that you agree) run your benchmarks
> with the patch applied?
>
> --
>
>     Manfred
>
>
>

> From fe2f257b1950a19bf5c6f67e71aa25c2f13bcdc3 Mon Sep 17 00:00:00 2001
> From: Manfred Spraul <manfred@colorfullife.com>
> Date: Sun, 24 May 2020 14:47:31 +0200
> Subject: [PATCH 2/2] ipc/msg.c: Handle case of senders not enqueuing the
> message
>
> The patch "ipc/msg.c: wake up senders until there is a queue empty
> capacity" avoids the thundering herd problem by wakeing up
> only as many potential senders as there is free space in the queue.
>
> This patch is a fix: If one of the senders doesn't enqueue its message,
> then a search for further potential senders must be performed.
>
> Signed-off-by: Manfred Spraul <manfred@colorfullife.com>
> ---
> ipc/msg.c | 21 +++++++++++++++++++++
> 1 file changed, 21 insertions(+)
>
> diff --git a/ipc/msg.c b/ipc/msg.c
> index 52d634b0a65a..f6d5188db38a 100644
> --- a/ipc/msg.c
> +++ b/ipc/msg.c
> @@ -208,6 +208,12 @@ static inline void ss_del(struct msg_sender *mss)
> list_del(&mss->list);
> }
>
> +/*
> + * ss_wakeup() assumes that the stored senders will enqueue the pending message.
> + * Thus: If a woken up task doesn't send the enqueued message for whatever
> + * reason, then that task must call ss_wakeup() again, to ensure that no
> + * wakeup is lost.
> + */
> static void ss_wakeup(struct msg_queue *msq,
> struct wake_q_head *wake_q, bool kill)
> {
> @@ -843,6 +849,7 @@ static long do_msgsnd(int msqid, long mtype, void __user *mtext,
> struct msg_queue *msq;
> struct msg_msg *msg;
> int err;
> + bool need_wakeup;
> struct ipc_namespace *ns;
> DEFINE_WAKE_Q(wake_q);
>
> @@ -869,6 +876,7 @@ static long do_msgsnd(int msqid, long mtype, void __user *mtext,
>
> ipc_lock_object(&msq->q_perm);
>
> + need_wakeup = false;
> for (;;) {
> struct msg_sender s;
>
> @@ -898,6 +906,13 @@ static long do_msgsnd(int msqid, long mtype, void __user *mtext,
> /* enqueue the sender and prepare to block */
> ss_add(msq, &s, msgsz);
>
> + /* Enqueuing a sender is actually an obligation:
> + * The sender must either enqueue the message, or call
> + * ss_wakeup(). Thus track that we have added our message
> + * to the candidates for the message queue.
> + */
> + need_wakeup = true;
> +
> if (!ipc_rcu_getref(&msq->q_perm)) {
> err = -EIDRM;
> goto out_unlock0;
> @@ -935,12 +950,18 @@ static long do_msgsnd(int msqid, long mtype, void __user *mtext,
> msq->q_qnum++;
> atomic_add(msgsz, &ns->msg_bytes);
> atomic_inc(&ns->msg_hdrs);
> +
> + /* we have fulfilled our obligation, no need for wakeup */
> + need_wakeup = false;
> }
>
> err = 0;
> msg = NULL;
>
> out_unlock0:
> + if (need_wakeup)
> + ss_wakeup(msq, &wake_q, false);
> +
> ipc_unlock_object(&msq->q_perm);
> wake_up_q(&wake_q);
> out_unlock1:
> --
> 2.26.2
>

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