From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755356AbaHUWci (ORCPT ); Thu, 21 Aug 2014 18:32:38 -0400 Received: from mail-gw1-out.broadcom.com ([216.31.210.62]:60671 "EHLO mail-gw1-out.broadcom.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754890AbaHUWch (ORCPT ); Thu, 21 Aug 2014 18:32:37 -0400 X-IronPort-AV: E=Sophos;i="5.04,376,1406617200"; d="scan'208";a="43522497" Subject: Re: [PATCH 1/3] tg3: Limit minimum tx queue wakeup threshold From: Michael Chan To: Benjamin Poirier CC: Prashant Sreedharan , , In-Reply-To: <20140821220424.GB7117@f1.synalogic.ca> References: <1408474371-19509-1-git-send-email-bpoirier@suse.de> <1408485647.7404.3.camel@LTIRV-MCHAN1.corp.ad.broadcom.com> <20140821220424.GB7117@f1.synalogic.ca> Content-Type: text/plain; charset="cp1252" Date: Thu, 21 Aug 2014 15:32:35 -0700 Message-ID: <1408660355.7644.50.camel@LTIRV-MCHAN1.corp.ad.broadcom.com> MIME-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 2014-08-21 at 15:04 -0700, Benjamin Poirier wrote: > On 2014/08/19 15:00, Michael Chan wrote: > > On Tue, 2014-08-19 at 11:52 -0700, Benjamin Poirier wrote: > > > diff --git a/drivers/net/ethernet/broadcom/tg3.c b/drivers/net/ethernet/broadcom/tg3.c > > > index 3ac5d23..b11c0fd 100644 > > > --- a/drivers/net/ethernet/broadcom/tg3.c > > > +++ b/drivers/net/ethernet/broadcom/tg3.c > > > @@ -202,7 +202,8 @@ static inline void _tg3_flag_clear(enum TG3_FLAGS flag, unsigned long *bits) > > > #endif > > > > > > /* minimum number of free TX descriptors required to wake up TX process */ > > > -#define TG3_TX_WAKEUP_THRESH(tnapi) ((tnapi)->tx_pending / 4) > > > +#define TG3_TX_WAKEUP_THRESH(tnapi) max_t(u32, (tnapi)->tx_pending / 4, \ > > > + MAX_SKB_FRAGS + 1) > > > > I think we should precompute this and store it in something like > > tp->tx_wake_thresh. > > I've tried this by adding the following patch at the end of the v2 > series but I did not measure a significant latency improvement. Was > there another reason for the change? Just performance. The wake up threshold is checked in the tx fast path in both start_xmit() and tg3_tx(). I would optimize such code for speed as much as possible. In the current code, it was just a right shift operation. Now, with max_t() added, I think I prefer having it pre-computed. The performance difference may not be measurable, but I think the compiled code size may be smaller too.