Re: Perf data with recent tg3 patches

["David S. Miller" <davem@xxxxxxxxxxxxx>]

From: Arthur Kepner <akepner@xxxxxxx>
Subject: Re: Perf data with recent tg3 patches
Date: Fri, 13 May 2005 16:57:51 -0700 (PDT)

> I found that the reason is that, 
> under high receive load, most of the time (~80%) the 
> tag in the status block changes between the time that 
> it's read (and saved as last_tag) in tg3_poll(), and when 
> it's written back to MAILBOX_INTERRUPT_0 in 
> tg3_restart_ints(). If I understand the way the status 
> tag works, that means that the card will immediately 
> generate another interrupt. That's consistent with 
> what I'm seeing - a much higher interrupt rate when the 
> tagged status patch is used.

Thanks for tracking this down.

Perhaps we can make the logic in tg3_poll() smarter about
this.  Something like:

        tg3_process_phy_events();
        tg3_tx();
        tg3_rx();

        if (tp->tg3_flags & TG3_FLAG_TAGGED_STATUS)
                tp->last_tag = sblk->status_tag;
        rmb();
        done = !tg3_has_work(tp);
        if (done) {
                spin_lock_irqsave(&tp->lock, flags);
                __netif_rx_complete(netdev);
                tg3_restart_ints(tp);
                spin_unlock_irqrestore(&tp->lock, flags);
        }
        return (done ? 0 : 1);

Basically, move the last_tag sample to after we do the
work, then recheck the RX/TX producer/consumer indexes.