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Re: [PATCH 2.6.9-rc2 7/8] S2io: NAPI fix

To: Raghavendra Koushik <raghavendra.koushik@xxxxxxxx>
Subject: Re: [PATCH 2.6.9-rc2 7/8] S2io: NAPI fix
From: Jeff Garzik <jgarzik@xxxxxxxxx>
Date: Fri, 15 Oct 2004 14:53:46 -0400
Cc: ravinandan.arakali@xxxxxxxx, "'Francois Romieu'" <romieu@xxxxxxxxxxxxx>, netdev@xxxxxxxxxxx, leonid.grossman@xxxxxxxx, rapuru.sriram@xxxxxxxx
In-reply-to: <200410141644.i9EGi839024175@guinness.s2io.com>
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Raghavendra Koushik wrote:
Hi Jeff,

My comments inline.


-----Original Message-----
From: Jeff Garzik [mailto:jgarzik@xxxxxxxxx]
Sent: Thursday, October 14, 2004 8:00 AM
To: ravinandan.arakali@xxxxxxxx
Cc: 'Francois Romieu'; netdev@xxxxxxxxxxx; leonid.grossman@xxxxxxxx;
raghavendra.koushik@xxxxxxxx; rapuru.sriram@xxxxxxxx
Subject: Re: [PATCH 2.6.9-rc2 7/8] S2io: NAPI fix

Comments on patch #7:

1) Can you prove that put_lock is really necessary, and not covered by
other methods of synchronization?  Typically the preferred model is that
your RX process requires _no_ spinlocks, and instead you use the net
stack API to ensure when your RX process is, and is not, running.


<KSK> Lets say, CPU0 after processing s2io_isr, schedules s2io_tasklet. The tasklet in turn calls fill_rx_buffer which will replenish skbs into Rx descriptors and start modifying the put index of the Rx ring, at the same time if there is another Rx interrupt being processed (rx_intr_handler) on CPU1, then it will try to read put index of the ring which can result in synchronization problem. So I created this new variable put_pos to track the absolute value of the put index and make sure all its accesses are between spin locks. The same problem cannot happen if NAPI is used since the s2io_poll and fill_rx_buffer calls are serialized.


Have you actually benchmarked this on SMP?

It seems to me that you pay a penalty cross-CPU traffic with this scheme?

        Jeff



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