dri-devel Archive on lore.kernel.org
 help / color / Atom feed
From: Thomas Gleixner <tglx@linutronix.de>
To: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>,
	LKML <linux-kernel@vger.kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>,
	Karthikeyan Mitran <m.karthikeyan@mobiveil.co.in>,
	Peter Zijlstra <peterz@infradead.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	dri-devel@lists.freedesktop.org,
	Chris Wilson <chris@chris-wilson.co.uk>,
	"James E.J. Bottomley" <James.Bottomley@HansenPartnership.com>,
	netdev@vger.kernel.org, Will Deacon <will@kernel.org>,
	Michal Simek <michal.simek@xilinx.com>,
	linux-s390@vger.kernel.org,
	afzal mohammed <afzal.mohd.ma@gmail.com>,
	Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>,
	Dave Jiang <dave.jiang@intel.com>,
	xen-devel@lists.xenproject.org, Leon Romanovsky <leon@kernel.org>,
	linux-rdma@vger.kernel.org, Marc Zyngier <maz@kernel.org>,
	Helge Deller <deller@gmx.de>,
	Russell King <linux@armlinux.org.uk>,
	Christian Borntraeger <borntraeger@de.ibm.com>,
	linux-pci@vger.kernel.org, Jakub Kicinski <kuba@kernel.org>,
	Heiko Carstens <hca@linux.ibm.com>,
	Wambui Karuga <wambui.karugax@gmail.com>,
	Allen Hubbe <allenbh@gmail.com>, Juergen Gross <jgross@suse.com>,
	intel-gfx@lists.freedesktop.org, linux-gpio@vger.kernel.org,
	Stefano Stabellini <sstabellini@kernel.org>,
	Rodrigo Vivi <rodrigo.vivi@intel.com>,
	Bjorn Helgaas <bhelgaas@google.com>,
	Lee Jones <lee.jones@linaro.org>,
	linux-arm-kernel@lists.infradead.org,
	Boris Ostrovsky <boris.ostrovsky@oracle.com>,
	David Airlie <airlied@linux.ie>,
	linux-parisc@vger.kernel.org,
	Pankaj Bharadiya <pankaj.laxminarayan.bharadiya@intel.com>,
	Hou Zhiqiang <Zhiqiang.Hou@nxp.com>,
	Tariq Toukan <tariqt@nvidia.com>, Jon Mason <jdmason@kudzu.us>,
	linux-ntb@googlegroups.com, Saeed Mahameed <saeedm@nvidia.com>,
	"David S. Miller" <davem@davemloft.net>
Subject: Re: [patch 14/30] drm/i915/pmu: Replace open coded kstat_irqs() copy
Date: Fri, 11 Dec 2020 13:57:49 +0100
Message-ID: <87y2i4h54i.fsf@nanos.tec.linutronix.de> (raw)
In-Reply-To: <ad05af1a-5463-2a80-0887-7629721d6863@linux.intel.com>

On Fri, Dec 11 2020 at 10:13, Tvrtko Ursulin wrote:
> On 10/12/2020 19:25, Thomas Gleixner wrote:

>> 
>> Aside of that the count is per interrupt line and therefore takes
>> interrupts from other devices into account which share the interrupt line
>> and are not handled by the graphics driver.
>> 
>> Replace it with a pmu private count which only counts interrupts which
>> originate from the graphics card.
>> 
>> To avoid atomics or heuristics of some sort make the counter field
>> 'unsigned long'. That limits the count to 4e9 on 32bit which is a lot and
>> postprocessing can easily deal with the occasional wraparound.
>
> After my failed hasty sketch from last night I had a different one which 
> was kind of heuristics based (re-reading the upper dword and retrying if 
> it changed on 32-bit).

The problem is that there will be two seperate modifications for the low
and high word. Several ways how the compiler can translate this, but the
problem is the same for all of them:

CPU 0                           CPU 1
        load low
        load high
        add  low, 1
        addc high, 0            
        store low               load high
--> NMI                         load low
                                load high and compare
        store high

You can't catch that. If this really becomes an issue you need a
sequence counter around it.
      

> But you are right - it is okay to at least start 
> like this today and if later there is a need we can either do that or 
> deal with wrap at PMU read time.

Right.

>> +/*
>> + * Interrupt statistic for PMU. Increments the counter only if the
>> + * interrupt originated from the the GPU so interrupts from a device which
>> + * shares the interrupt line are not accounted.
>> + */
>> +static inline void pmu_irq_stats(struct drm_i915_private *priv,
>
> We never use priv as a local name, it should be either i915 or
> dev_priv.

Sure, will fix.

>> +	/*
>> +	 * A clever compiler translates that into INC. A not so clever one
>> +	 * should at least prevent store tearing.
>> +	 */
>> +	WRITE_ONCE(priv->pmu.irq_count, priv->pmu.irq_count + 1);
>
> Curious, probably more educational for me - given x86_32 and x86_64, and 
> the context of it getting called, what is the difference from just doing 
> irq_count++?

Several reasons:

    1) The compiler can pretty much do what it wants with cnt++
       including tearing and whatever. https://lwn.net/Articles/816850/
       for the full set of insanities.

       Not really a problem here, but

    2) It's annotating the reader and the writer side and documenting
       that this is subject to concurrency

    3) It will prevent KCSAN to complain about the data race,
       i.e. concurrent modification while reading.

Thanks,

        tglx

>> --- a/drivers/gpu/drm/i915/i915_pmu.c
>> +++ b/drivers/gpu/drm/i915/i915_pmu.c
>> @@ -423,22 +423,6 @@ static enum hrtimer_restart i915_sample(
>>   	return HRTIMER_RESTART;
>>   }
>
> In this file you can also drop the #include <linux/irq.h> line.

Indeed.

Thanks,

        tglx
_______________________________________________
dri-devel mailing list
dri-devel@lists.freedesktop.org
https://lists.freedesktop.org/mailman/listinfo/dri-devel

  reply index

Thread overview: 73+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-12-10 19:25 [patch 00/30] genirq: Treewide hunt for irq descriptor abuse and assorted fixes Thomas Gleixner
2020-12-10 19:25 ` [patch 01/30] genirq: Move irq_has_action() into core code Thomas Gleixner
2020-12-10 19:25 ` [patch 02/30] genirq: Move status flag checks to core Thomas Gleixner
2020-12-27 19:20   ` Guenter Roeck
2021-01-11 10:14     ` Thomas Gleixner
2020-12-10 19:25 ` [patch 03/30] genirq: Move irq_set_lockdep_class() " Thomas Gleixner
2020-12-11 17:53   ` Andy Shevchenko
2020-12-11 21:08     ` Thomas Gleixner
2020-12-11 22:07       ` Thomas Gleixner
2020-12-12 13:22         ` Andy Shevchenko
2020-12-10 19:25 ` [patch 04/30] genirq: Provide irq_get_effective_affinity() Thomas Gleixner
2020-12-10 19:25 ` [patch 05/30] genirq: Annotate irq stats data races Thomas Gleixner
2020-12-10 19:25 ` [patch 06/30] parisc/irq: Simplify irq count output for /proc/interrupts Thomas Gleixner
2020-12-10 19:25 ` [patch 07/30] genirq: Make kstat_irqs() static Thomas Gleixner
2020-12-10 19:25 ` [patch 08/30] genirq: Provide kstat_irqdesc_cpu() Thomas Gleixner
2020-12-10 19:25 ` [patch 09/30] ARM: smp: Use irq_desc_kstat_cpu() in show_ipi_list() Thomas Gleixner
2020-12-11 18:08   ` Marc Zyngier
2020-12-10 19:25 ` [patch 10/30] arm64/smp: Use irq_desc_kstat_cpu() in arch_show_interrupts() Thomas Gleixner
2020-12-11 18:08   ` Marc Zyngier
2020-12-10 19:25 ` [patch 11/30] parisc/irq: Use irq_desc_kstat_cpu() in show_interrupts() Thomas Gleixner
2020-12-10 19:25 ` [patch 12/30] s390/irq: Use irq_desc_kstat_cpu() in show_msi_interrupt() Thomas Gleixner
2020-12-10 20:31   ` Heiko Carstens
2020-12-10 19:25 ` [patch 13/30] drm/i915/lpe_audio: Remove pointless irq_to_desc() usage Thomas Gleixner
2020-12-10 19:48   ` [Intel-gfx] " Ville Syrjälä
2020-12-11  9:51     ` Jani Nikula
2020-12-10 19:25 ` [patch 14/30] drm/i915/pmu: Replace open coded kstat_irqs() copy Thomas Gleixner
2020-12-11  9:54   ` Jani Nikula
2020-12-11 10:13   ` Tvrtko Ursulin
2020-12-11 12:57     ` Thomas Gleixner [this message]
2020-12-11 14:19       ` David Laight
2020-12-11 21:10         ` Thomas Gleixner
2020-12-11 22:06           ` David Laight
2020-12-10 19:25 ` [patch 15/30] pinctrl: nomadik: Use irq_has_action() Thomas Gleixner
2020-12-12  0:45   ` Linus Walleij
2020-12-10 19:25 ` [patch 16/30] mfd: ab8500-debugfs: Remove the racy fiddling with irq_desc Thomas Gleixner
2020-12-11  8:22   ` Linus Walleij
2020-12-11 10:04   ` Lee Jones
2020-12-11 18:12   ` Andy Shevchenko
2020-12-10 19:25 ` [patch 17/30] NTB/msi: Use irq_has_action() Thomas Gleixner
2020-12-10 20:33   ` Logan Gunthorpe
2020-12-10 19:25 ` [patch 18/30] PCI: xilinx-nwl: Use irq_data_get_irq_chip_data() Thomas Gleixner
2020-12-10 22:56   ` Rob Herring
2020-12-10 19:25 ` [patch 19/30] PCI: mobiveil: " Thomas Gleixner
2020-12-10 22:56   ` Rob Herring
2020-12-10 19:25 ` [patch 20/30] net/mlx4: Replace irq_to_desc() abuse Thomas Gleixner
2020-12-13 11:24   ` Tariq Toukan
2020-12-10 19:25 ` [patch 21/30] net/mlx4: Use effective interrupt affinity Thomas Gleixner
2020-12-13 11:31   ` Tariq Toukan
2020-12-10 19:25 ` [patch 22/30] net/mlx5: Replace irq_to_desc() abuse Thomas Gleixner
2020-12-13 11:34   ` Tariq Toukan
2020-12-14 21:13   ` Saeed Mahameed
2020-12-10 19:25 ` [patch 23/30] net/mlx5: Use effective interrupt affinity Thomas Gleixner
2020-12-13 11:35   ` Tariq Toukan
2020-12-14 20:58   ` Saeed Mahameed
2020-12-10 19:26 ` [patch 24/30] xen/events: Remove unused bind_evtchn_to_irq_lateeoi() Thomas Gleixner
2020-12-10 23:19   ` boris.ostrovsky
2020-12-11  0:04     ` Thomas Gleixner
2020-12-10 19:26 ` [patch 25/30] xen/events: Remove disfunct affinity spreading Thomas Gleixner
2020-12-10 19:26 ` [patch 26/30] xen/events: Use immediate affinity setting Thomas Gleixner
2020-12-10 19:26 ` [patch 27/30] xen/events: Only force affinity mask for percpu interrupts Thomas Gleixner
2020-12-10 23:20   ` boris.ostrovsky
2020-12-11  0:06     ` Thomas Gleixner
2020-12-11  6:17     ` Jürgen Groß
     [not found]       ` <874kksiras.fsf@nanos.tec.linutronix.de>
2020-12-11 10:23         ` Jürgen Groß
2020-12-11 12:10     ` Jürgen Groß
2020-12-11 12:37       ` Thomas Gleixner
2020-12-11 14:29         ` boris.ostrovsky
2020-12-11 21:27           ` Thomas Gleixner
2020-12-11 22:21             ` Andrew Cooper
2020-12-11 22:56               ` Thomas Gleixner
2020-12-10 19:26 ` [patch 28/30] xen/events: Reduce irq_info::spurious_cnt storage size Thomas Gleixner
2020-12-10 19:26 ` [patch 29/30] xen/events: Implement irq distribution Thomas Gleixner
2020-12-10 19:26 ` [patch 30/30] genirq: Remove export of irq_to_desc() Thomas Gleixner

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=87y2i4h54i.fsf@nanos.tec.linutronix.de \
    --to=tglx@linutronix.de \
    --cc=James.Bottomley@HansenPartnership.com \
    --cc=Zhiqiang.Hou@nxp.com \
    --cc=afzal.mohd.ma@gmail.com \
    --cc=airlied@linux.ie \
    --cc=allenbh@gmail.com \
    --cc=bhelgaas@google.com \
    --cc=boris.ostrovsky@oracle.com \
    --cc=borntraeger@de.ibm.com \
    --cc=catalin.marinas@arm.com \
    --cc=chris@chris-wilson.co.uk \
    --cc=dave.jiang@intel.com \
    --cc=davem@davemloft.net \
    --cc=deller@gmx.de \
    --cc=dri-devel@lists.freedesktop.org \
    --cc=hca@linux.ibm.com \
    --cc=intel-gfx@lists.freedesktop.org \
    --cc=jdmason@kudzu.us \
    --cc=jgross@suse.com \
    --cc=kuba@kernel.org \
    --cc=lee.jones@linaro.org \
    --cc=leon@kernel.org \
    --cc=linux-arm-kernel@lists.infradead.org \
    --cc=linux-gpio@vger.kernel.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linux-ntb@googlegroups.com \
    --cc=linux-parisc@vger.kernel.org \
    --cc=linux-pci@vger.kernel.org \
    --cc=linux-rdma@vger.kernel.org \
    --cc=linux-s390@vger.kernel.org \
    --cc=linux@armlinux.org.uk \
    --cc=lorenzo.pieralisi@arm.com \
    --cc=m.karthikeyan@mobiveil.co.in \
    --cc=mark.rutland@arm.com \
    --cc=maz@kernel.org \
    --cc=michal.simek@xilinx.com \
    --cc=netdev@vger.kernel.org \
    --cc=pankaj.laxminarayan.bharadiya@intel.com \
    --cc=peterz@infradead.org \
    --cc=rodrigo.vivi@intel.com \
    --cc=saeedm@nvidia.com \
    --cc=sstabellini@kernel.org \
    --cc=tariqt@nvidia.com \
    --cc=tvrtko.ursulin@linux.intel.com \
    --cc=wambui.karugax@gmail.com \
    --cc=will@kernel.org \
    --cc=xen-devel@lists.xenproject.org \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

dri-devel Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/dri-devel/0 dri-devel/git/0.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 dri-devel dri-devel/ https://lore.kernel.org/dri-devel \
		dri-devel@lists.freedesktop.org
	public-inbox-index dri-devel

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.freedesktop.lists.dri-devel


AGPL code for this site: git clone https://public-inbox.org/public-inbox.git