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

On 5/26/20 9:56 AM, Artur Barsegyan wrote:
> 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.

I don't get your concern, or perhaps this is a feature that I had always
assumed as "normal":

"msg_fits_inqueue(msq, msgsz)" is in the loop, this ensures progress.

The rational is a design decision:

The check for pipeline sending is only done if there would be space to
store the message in the queue.

I was afraid that performing the pipeline send immediately, without
checking queue availability, could break apps:

Some messages would arrive immediately (if there is a waiting receiver),
other messages are stuck forever (since the queue is full).

Initial patch: https://lkml.org/lkml/1999/10/3/5 (without any remarks
about the design decision)

The risk that I had seen was theoretical, I do not have any real bug
reports. So we could change it.

Perhaps: Go in the same direction as it was done for POSIX mqueue:
implement pipelined receive.

> 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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