Date: Thu, 29 Jun 2017 13:58:22 -0700
From: Jakub Kicinski <jakub.kicinski@netronome.com>
To: "Luis R. Rodriguez" <mcgrof@kernel.org>
Cc: nbroeking@me.com, Linus Torvalds <torvalds@linux-foundation.org>,
        Davidlohr Bueso <dave@stgolabs.net>,
        Thomas Gleixner <tglx@linutronix.de>,
        Greg KH <gregkh@linuxfoundation.org>, mfuzzey@parkeon.com,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Dmitry Torokhov <dmitry.torokhov@gmail.com>,
        Daniel Wagner <wagi@monom.org>, David Woodhouse <dwmw2@infradead.org>,
        jewalt@lgsinnovations.com, rafal@milecki.pl,
        Arend Van Spriel <arend.vanspriel@broadcom.com>,
        "Rafael J. Wysocki" <rjw@rjwysocki.net>,
        "Li, Yi" <yi1.li@linux.intel.com>, atull@kernel.org,
        Moritz Fischer <moritz.fischer@ettus.com>,
        Petr Mladek <pmladek@suse.com>,
        Johannes Berg <johannes.berg@intel.com>,
        Emmanuel Grumbach <emmanuel.grumbach@intel.com>,
        "Coelho, Luciano" <luciano.coelho@intel.com>,
        Kalle Valo <kvalo@codeaurora.org>, Andrew Lutomirski <luto@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        "AKASHI, Takahiro" <takahiro.akashi@linaro.org>,
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        Hans de Goede <hdegoede@redhat.com>, Alan Cox <alan@linux.intel.com>,
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        Michael Kerrisk <mtk.manpages@gmail.com>,
        Paul Gortmaker <paul.gortmaker@windriver.com>,
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        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "stable # 4 . 6" <stable@vger.kernel.org>
Subject: Re: [PATCH 2/4] swait: add the missing killable swaits
Message-ID: <20170629135822.366fd67a@cakuba.netronome.com>
In-Reply-To: <20170629194455.GR21846@wotan.suse.de>
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On Thu, 29 Jun 2017 21:44:55 +0200, Luis R. Rodriguez wrote:
> On Thu, Jun 29, 2017 at 09:40:15PM +0200, Luis R. Rodriguez wrote:
> > On Thu, Jun 29, 2017 at 11:59:29AM -0700, Linus Torvalds wrote:  
> > > On Thu, Jun 29, 2017 at 11:33 AM, Davidlohr Bueso <dave@stgolabs.net> wrote:  
> > > > On Thu, 29 Jun 2017, Linus Torvalds wrote:
> > > >  
> > > >> I actually think swait is pure garbage. Most users only wake up one
> > > >> process anyway, and using swait for that is stupid. If you only wake
> > > >> up one, you might as well just have a single process pointer, not a
> > > >> wait list at all, and then use "wake_up_process()".  
> > > >
> > > > But you still need the notion of a queue, even if you wake one task
> > > > at a time... I'm probably missing your point here.  
> > > 
> > > The *reason* they wake up only one seems to be that there really is
> > > just one. It's some per-cpu idle thread for kvm, and for RCU it's the
> > > RCU workqueue thread.
> > > 
> > > So the queue literally looks suspiciously pointless.
> > > 
> > > But I might be wrong, and there can actually be multiple entries.  
> > 
> > Since this swake_up() --> swake_up_all() reportedly *fixed* the one wake up
> > issue it would seem this does queue [0]. That said, I don't see any simple tests
> > tools/testing/selftests/swait but then again we don't have test for regular
> > waits either...
> > 
> > [0] https://bugzilla.kernel.org/show_bug.cgi?id=195477  
> 
> I should also note that the swake_up_all() should have only helped in cases where
> 3 cards were used, as if only 2 were used that should have been covered by just
> the swake_up(). Unless of course I hear otherwise by the reporter, Nicolas or
> from Jakub.

I was hitting this with 2 cards.