From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-8.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING, SPF_HELO_NONE,SPF_PASS autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 50979C433DB for ; Tue, 23 Feb 2021 05:26:57 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 0539764E31 for ; Tue, 23 Feb 2021 05:26:56 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230396AbhBWF01 (ORCPT ); Tue, 23 Feb 2021 00:26:27 -0500 Received: from kvm5.telegraphics.com.au ([98.124.60.144]:43600 "EHLO kvm5.telegraphics.com.au" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230014AbhBWF0C (ORCPT ); Tue, 23 Feb 2021 00:26:02 -0500 Received: from localhost (localhost.localdomain [127.0.0.1]) by kvm5.telegraphics.com.au (Postfix) with ESMTP id A09202A0A3; Tue, 23 Feb 2021 00:25:16 -0500 (EST) Date: Tue, 23 Feb 2021 16:25:15 +1100 (AEDT) From: Finn Thain To: "Song Bao Hua (Barry Song)" cc: tanxiaofei , "jejb@linux.ibm.com" , "martin.petersen@oracle.com" , "linux-scsi@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "linuxarm@openeuler.org" , "linux-m68k@vger.kernel.org" Subject: RE: [Linuxarm] Re: [PATCH for-next 00/32] spin lock usage optimization for SCSI drivers In-Reply-To: <8c99b5c060eb4e5aa5b604666a8db516@hisilicon.com> Message-ID: References: <1612697823-8073-1-git-send-email-tanxiaofei@huawei.com> <31cd807d-3d0-ed64-60d-fde32cb3833c@telegraphics.com.au> <7bc39d19-f4cc-8028-11e6-c0e45421a765@huawei.com> <588a87f-ae42-0b7-749e-c780ce5c3e4f@telegraphics.com.au> <8c99b5c060eb4e5aa5b604666a8db516@hisilicon.com> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, 22 Feb 2021, Song Bao Hua (Barry Song) wrote: > > On Thu, 18 Feb 2021, Xiaofei Tan wrote: > > > > > On 2021/2/9 13:06, Finn Thain wrote: > > > > On Tue, 9 Feb 2021, Song Bao Hua (Barry Song) wrote: > > > > > > > > > > On Sun, 7 Feb 2021, Xiaofei Tan wrote: > > > > > > > > > > > > > Replace spin_lock_irqsave with spin_lock in hard IRQ of SCSI > > > > > > > drivers. There are no function changes, but may speed up if > > > > > > > interrupt happen too often. > > > > > > > > > > > > This change doesn't necessarily work on platforms that support > > > > > > nested interrupts. > > > > > > > > > > > > Were you able to measure any benefit from this change on some > > > > > > other platform? > > > > > > > > > > I think the code disabling irq in hardIRQ is simply wrong. Since > > > > > this commit > > > > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=e58aa3d2d0cc > > > > > genirq: Run irq handlers with interrupts disabled > > > > > > > > > > interrupt handlers are definitely running in a irq-disabled > > > > > context unless irq handlers enable them explicitly in the > > > > > handler to permit other interrupts. > > > > > > > > > > > > > Repeating the same claim does not somehow make it true. If you put > > > > your claim to the test, you'll see that that interrupts are not > > > > disabled on m68k when interrupt handlers execute. > > > > > > > > The Interrupt Priority Level (IPL) can prevent any given irq > > > > handler from being re-entered, but an irq with a higher priority > > > > level may be handled during execution of a lower priority irq > > > > handler. > > > > > > > > sonic_interrupt() uses an irq lock within an interrupt handler to > > > > avoid issues relating to this. This kind of locking may be needed > > > > in the drivers you are trying to patch. Or it might not. > > > > Apparently, no-one has looked. > > > > > > > > > > According to your discussion with Barry, it seems that m68k is a > > > little different from other architecture, and this kind of > > > modification of this patch cannot be applied to m68k. So, could help > > > to point out which driver belong to m68k architecture in this patch > > > set of SCSI? I can remove them. > > > > > > > If you would claim that "there are no function changes" in your > > patches (as above) then the onus is on you to support that claim. > > > > I assume that there are some platforms on which your assumptions hold. > > > > With regard to drivers for those platforms, you might want to explain > > why your patches should be applied there, given that the existing code > > is superior for being more portable. > > I don't think it has nothing to do with portability. In the case of > sonic_interrupt() you pointed out, on m68k, there is a high-priority > interrupt can preempt low-priority interrupt, they will result in access > the same critical data. M68K's spin_lock_irqsave() can disable the > high-priority interrupt and avoid the race condition of the data. So the > case should not be touched. I'd like to accept the reality and leave > sonic_interrupt() alone. > > However, even on m68k, spin_lock_irqsave is not needed for other > ordinary cases. > If there is no other irq handler coming to access same critical data, > it is pointless to hold a redundant irqsave lock in irqhandler even > on m68k. > > In thread contexts, we always need that if an irqhandler can preempt > those threads and access the same data. In hardirq, if there is an > high-priority which can jump out on m68k to access the critical data > which needs protection, we use the spin_lock_irqsave as you have used in > sonic_interrupt(). Otherwise, the irqsave is also redundant for m68k. > > > > > > BTW, sonic_interrupt() is from net driver natsemi, right? It would > > > be appreciative if only discuss SCSI drivers in this patch set. > > > thanks. > > > > > > > The 'net' subsystem does have some different requirements than the > > 'scsi' subsystem. But I don't see how that's relevant. Perhaps you can > > explain it. Thanks. > > The difference is that if there are two co-existing interrupts which can > access the same critical data on m68k. I don't think net and scsi > matter. What really matters is the specific driver. > Regarding m68k, your analysis overlooks the timing issue. E.g. patch 11/32 could be a problem because removing the irqsave would allow PDMA transfers to be interrupted. Aside from the timing issues, I agree with your analysis above regarding m68k. With regard to other architectures and platforms, in specific cases, e.g. where there's never more than one IRQ involved, then I could agree that your assumptions probably hold and an irqsave would be probably redundant. When you find a redundant irqsave, to actually patch it would bring a risk of regression with little or no reward. It's not my place to veto this entire patch series on that basis but IMO this kind of churn is misguided. > Thanks > Barry > >