Re: [PATCH 2/2] qdisc_restart - couple of optimizations.
On Wed, Jun 13, 2007 at 02:10:49PM +0530, Krishna Kumar wrote:
- BUG_ON((int) q-q.qlen 0) was a relic from old times when -1
meant more packets are available, and __qdisc_run used to loop
when qdisc_restart() returned -1. During those days, it was
necessary to make sure that qlen is never less than zero, since
__qdisc_run would get into an infinite loop if no packets are on
the queue and this bug in qdisc was there (and worse - no more
skbs could ever get queue'd as we hold the queue lock too). With
Herbert's recent change to return values, this check is not
required. Hopefully Herbert can validate this change. If at all
this is required, it should be added to skb_dequeue (in failure
case), and not to qdisc_qlen.
Yes I agree that this check is no longer critical.
Cheers,
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Re: [PATCH 2/2] qdisc_restart - couple of optimizations.
On Wed, Jun 13, 2007 at 10:51:00AM -0700, Waskiewicz Jr, Peter P wrote:
I somewhat disagree here. The underlying driver can conceivably stop
the device queue even if the stack holds the queue lock during an
interrupt to clean Tx descriptors, and it finds it's out of them or
needs to grab the device for whatever reason. Granted this is a corner
case, and the net effect would be a simple requeue of the skb, but
checking the status of the queue at the last possible moment before
entering the driver could alleviate the requeue in the time between
-dequeue() from the qdisc, and hard_start_xmit() if an event like I
mentioned happened.
IMHO this scenario occurs so infrequently that the check isn't worth it
especially since the driver has to be able to deal with us calling it
after netif_stop_queue() anyway.
Cheers,
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RE: [PATCH 2/2] qdisc_restart - couple of optimizations.
IMHO this scenario occurs so infrequently that the check isn't worth it especially since the driver has to be able to deal with us calling it after netif_stop_queue() anyway. That sounds just fine to me. Thanks Krishna and Herbert for weighing in on this. -PJ Waskiewicz - To unsubscribe from this list: send the line unsubscribe netdev in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html
RE: [PATCH 2/2] qdisc_restart - couple of optimizations.
- netif_queue_stopped need not be called inside qdisc_restart as it has been called already in qdisc_run() before the first skb is sent, and in __qdisc_run() after each intermediate skb is sent (note : we are the only sender, so the queue cannot get stopped while the tx lock was got in the ~LLTX case). I somewhat disagree here. The underlying driver can conceivably stop the device queue even if the stack holds the queue lock during an interrupt to clean Tx descriptors, and it finds it's out of them or needs to grab the device for whatever reason. Granted this is a corner case, and the net effect would be a simple requeue of the skb, but checking the status of the queue at the last possible moment before entering the driver could alleviate the requeue in the time between -dequeue() from the qdisc, and hard_start_xmit() if an event like I mentioned happened. I'm ok with it either way, especially since this is a corner case. But it does need to be considered that it can happen. Cheers, -PJ Waskiewicz - To unsubscribe from this list: send the line unsubscribe netdev in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html
RE: [PATCH 2/2] qdisc_restart - couple of optimizations.
Hi Peter, Thanks for your feedback. - netif_queue_stopped need not be called inside qdisc_restart as it has been called already in qdisc_run() before the first skb is sent, and in __qdisc_run() after each intermediate skb is sent (note : we are the only sender, so the queue cannot get stopped while the tx lock was got in the ~LLTX case). I somewhat disagree here. The underlying driver can conceivably stop the device queue even if the stack holds the queue lock during an interrupt to clean Tx descriptors, and it finds it's out of them or needs to grab the device for whatever reason. Granted this is a corner case, and the net effect would be a simple requeue of the skb, but checking the status of the queue at the last possible moment before entering the driver could alleviate the requeue in the time between -dequeue() from the qdisc, and hard_start_xmit() if an event like I mentioned happened. After seeing a few drivers, I understand that the rx intr (to clean TX descriptors) can only enable the queue and not stop the queue (as it will normally not queue any packets and only clean up the sent ones). I don't find any way that the driver can stop the queue once the top layer determines it is OK to send and is enqueing. It is a wasted check for almost every packet (and IMHO opinion, for every packet). And as you said - if a driver were written differently to stop the queue even in the clean path, then that rare event (should be very rare as we checked for stop_queue just a few instrutions earlier) will result in a requeue, but the normal path is not penalized. Thanks, - KK I'm ok with it either way, especially since this is a corner case. But it does need to be considered that it can happen. - To unsubscribe from this list: send the line unsubscribe netdev in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html
