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From: Stefan Hajnoczi <stefanha@redhat.com>
To: "Sohil Mehta" <sohil.mehta@intel.com>,
	"Peter Maydell" <peter.maydell@linaro.org>,
	"Alex Bennée" <alex.bennee@linaro.org>,
	"Richard Henderson" <richard.henderson@linaro.org>
Cc: Andy Lutomirski <luto@kernel.org>,
	the arch/x86 maintainers <x86@kernel.org>,
	Tony Luck <tony.luck@intel.com>,
	Dave Hansen <dave.hansen@intel.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	"H. Peter Anvin" <hpa@zytor.com>, Jens Axboe <axboe@kernel.dk>,
	Christian Brauner <christian@brauner.io>,
	"Peter Zijlstra (Intel)" <peterz@infradead.org>,
	Shuah Khan <shuah@kernel.org>, Arnd Bergmann <arnd@arndb.de>,
	Jonathan Corbet <corbet@lwn.net>, Raj Ashok <ashok.raj@intel.com>,
	Jacob Pan <jacob.jun.pan@linux.intel.com>,
	Gayatri Kammela <gayatri.kammela@intel.com>,
	Zeng Guang <guang.zeng@intel.com>,
	"Williams, Dan J" <dan.j.williams@intel.com>,
	Randy E Witt <randy.e.witt@intel.com>,
	"Shankar, Ravi V" <ravi.v.shankar@intel.com>,
	Ramesh Thomas <ramesh.thomas@intel.com>,
	Linux API <linux-api@vger.kernel.org>,
	linux-arch@vger.kernel.org,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	linux-kselftest@vger.kernel.org, qemu-devel@nongnu.org
Subject: Re: [RFC PATCH 00/13] x86 User Interrupts support
Date: Fri, 1 Oct 2021 17:35:58 +0100	[thread overview]
Message-ID: <YVc47ohb4nxrBO5h@stefanha-x1.localdomain> (raw)
In-Reply-To: <778d40fe-ad8e-fd7c-4caa-499910bb0925@intel.com>

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On Thu, Sep 30, 2021 at 10:24:24AM -0700, Sohil Mehta wrote:
> 
> On 9/30/2021 9:30 AM, Stefan Hajnoczi wrote:
> > On Tue, Sep 28, 2021 at 09:31:34PM -0700, Andy Lutomirski wrote:
> > > 
> > > I spent some time reviewing the docs (ISE) and contemplating how this all fits together, and I have a high level question:
> > > 
> > > Can someone give an example of a realistic workload that would benefit from SENDUIPI and precisely how it would use SENDUIPI?  Or an example of a realistic workload that would benefit from hypothetical device-initiated user interrupts and how it would use them?  I'm having trouble imagining something that wouldn't work as well or better by simply polling, at least on DMA-coherent architectures like x86.
> > I was wondering the same thing. One thing came to mind:
> > 
> > An application that wants to be *interrupted* from what it's doing
> > rather than waiting until the next polling point. For example,
> > applications that are CPU-intensive and have green threads. I can't name
> > a real application like this though :P.
> 
> Thank you Stefan and Andy for giving this some thought.
> 
> We are consolidating the information internally on where and how exactly we
> expect to see benefits with real workloads for the various sources of User
> Interrupts. It will take a few days to get back on this one.

One possible use case came to mind in QEMU's TCG just-in-time compiler:

QEMU's TCG threads execute translated code. There are events that
require interrupting these threads. Today a check is performed at the
start of every translated block. Most of the time the check is false and
it's a waste of CPU.

User interrupts can eliminate the need for checks by interrupting TCG
threads when events occur.

I don't know whether this will improve performance or how feasible it is
to implement, but I've added people who might have ideas. (For a summary
of user interrupts, see
https://lwn.net/SubscriberLink/871113/60652640e11fc5df/.)

Stefan

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WARNING: multiple messages have this Message-ID (diff)
From: Stefan Hajnoczi <stefanha@redhat.com>
To: "Sohil Mehta" <sohil.mehta@intel.com>,
	"Peter Maydell" <peter.maydell@linaro.org>,
	"Alex Bennée" <alex.bennee@linaro.org>,
	"Richard Henderson" <richard.henderson@linaro.org>
Cc: "Peter Zijlstra \(Intel\)" <peterz@infradead.org>,
	qemu-devel@nongnu.org, Dave Hansen <dave.hansen@intel.com>,
	linux-kselftest@vger.kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
	Shuah Khan <shuah@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	linux-arch@vger.kernel.org, Raj Ashok <ashok.raj@intel.com>,
	Jonathan Corbet <corbet@lwn.net>,
	the arch/x86 maintainers <x86@kernel.org>,
	Ingo Molnar <mingo@redhat.com>, Zeng Guang <guang.zeng@intel.com>,
	Gayatri Kammela <gayatri.kammela@intel.com>,
	"Shankar, Ravi V" <ravi.v.shankar@intel.com>,
	Jacob Pan <jacob.jun.pan@linux.intel.com>,
	Arnd Bergmann <arnd@arndb.de>,
	Ramesh Thomas <ramesh.thomas@intel.com>,
	Borislav Petkov <bp@alien8.de>, Andy Lutomirski <luto@kernel.org>,
	"Williams, Dan J" <dan.j.williams@intel.com>,
	Christian Brauner <christian@brauner.io>,
	Jens Axboe <axboe@kernel.dk>, Tony Luck <tony.luck@intel.com>,
	Linux API <linux-api@vger.kernel.org>,
	Randy E Witt <randy.e.witt@intel.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [RFC PATCH 00/13] x86 User Interrupts support
Date: Fri, 1 Oct 2021 17:35:58 +0100	[thread overview]
Message-ID: <YVc47ohb4nxrBO5h@stefanha-x1.localdomain> (raw)
In-Reply-To: <778d40fe-ad8e-fd7c-4caa-499910bb0925@intel.com>

[-- Attachment #1: Type: text/plain, Size: 2018 bytes --]

On Thu, Sep 30, 2021 at 10:24:24AM -0700, Sohil Mehta wrote:
> 
> On 9/30/2021 9:30 AM, Stefan Hajnoczi wrote:
> > On Tue, Sep 28, 2021 at 09:31:34PM -0700, Andy Lutomirski wrote:
> > > 
> > > I spent some time reviewing the docs (ISE) and contemplating how this all fits together, and I have a high level question:
> > > 
> > > Can someone give an example of a realistic workload that would benefit from SENDUIPI and precisely how it would use SENDUIPI?  Or an example of a realistic workload that would benefit from hypothetical device-initiated user interrupts and how it would use them?  I'm having trouble imagining something that wouldn't work as well or better by simply polling, at least on DMA-coherent architectures like x86.
> > I was wondering the same thing. One thing came to mind:
> > 
> > An application that wants to be *interrupted* from what it's doing
> > rather than waiting until the next polling point. For example,
> > applications that are CPU-intensive and have green threads. I can't name
> > a real application like this though :P.
> 
> Thank you Stefan and Andy for giving this some thought.
> 
> We are consolidating the information internally on where and how exactly we
> expect to see benefits with real workloads for the various sources of User
> Interrupts. It will take a few days to get back on this one.

One possible use case came to mind in QEMU's TCG just-in-time compiler:

QEMU's TCG threads execute translated code. There are events that
require interrupting these threads. Today a check is performed at the
start of every translated block. Most of the time the check is false and
it's a waste of CPU.

User interrupts can eliminate the need for checks by interrupting TCG
threads when events occur.

I don't know whether this will improve performance or how feasible it is
to implement, but I've added people who might have ideas. (For a summary
of user interrupts, see
https://lwn.net/SubscriberLink/871113/60652640e11fc5df/.)

Stefan

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  parent reply	other threads:[~2021-10-01 16:36 UTC|newest]

Thread overview: 89+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-09-13 20:01 [RFC PATCH 00/13] x86 User Interrupts support Sohil Mehta
2021-09-13 20:01 ` [RFC PATCH 01/13] x86/uintr/man-page: Include man pages draft for reference Sohil Mehta
2021-09-13 20:01 ` [RFC PATCH 02/13] Documentation/x86: Add documentation for User Interrupts Sohil Mehta
2021-09-13 20:01 ` [RFC PATCH 03/13] x86/cpu: Enumerate User Interrupts support Sohil Mehta
2021-09-23 22:24   ` Thomas Gleixner
2021-09-24 19:59     ` Sohil Mehta
2021-09-27 20:42     ` Sohil Mehta
2021-09-13 20:01 ` [RFC PATCH 04/13] x86/fpu/xstate: Enumerate User Interrupts supervisor state Sohil Mehta
2021-09-23 22:34   ` Thomas Gleixner
2021-09-27 22:25     ` Sohil Mehta
2021-09-13 20:01 ` [RFC PATCH 05/13] x86/irq: Reserve a user IPI notification vector Sohil Mehta
2021-09-23 23:07   ` Thomas Gleixner
2021-09-25 13:30     ` Thomas Gleixner
2021-09-26 12:39       ` Thomas Gleixner
2021-09-27 19:07         ` Sohil Mehta
2021-09-28  8:11           ` Thomas Gleixner
2021-09-27 19:26     ` Sohil Mehta
2021-09-13 20:01 ` [RFC PATCH 06/13] x86/uintr: Introduce uintr receiver syscalls Sohil Mehta
2021-09-23 12:26   ` Greg KH
2021-09-24  0:05     ` Thomas Gleixner
2021-09-27 23:20     ` Sohil Mehta
2021-09-28  4:39       ` Greg KH
2021-09-28 16:47         ` Sohil Mehta
2021-09-23 23:52   ` Thomas Gleixner
2021-09-27 23:57     ` Sohil Mehta
2021-09-13 20:01 ` [RFC PATCH 07/13] x86/process/64: Add uintr task context switch support Sohil Mehta
2021-09-24  0:41   ` Thomas Gleixner
2021-09-28  0:30     ` Sohil Mehta
2021-09-13 20:01 ` [RFC PATCH 08/13] x86/process/64: Clean up uintr task fork and exit paths Sohil Mehta
2021-09-24  1:02   ` Thomas Gleixner
2021-09-28  1:23     ` Sohil Mehta
2021-09-13 20:01 ` [RFC PATCH 09/13] x86/uintr: Introduce vector registration and uintr_fd syscall Sohil Mehta
2021-09-24 10:33   ` Thomas Gleixner
2021-09-28 20:40     ` Sohil Mehta
2021-09-13 20:01 ` [RFC PATCH 10/13] x86/uintr: Introduce user IPI sender syscalls Sohil Mehta
2021-09-23 12:28   ` Greg KH
2021-09-28 18:01     ` Sohil Mehta
2021-09-29  7:04       ` Greg KH
2021-09-29 14:27         ` Sohil Mehta
2021-09-24 10:54   ` Thomas Gleixner
2021-09-13 20:01 ` [RFC PATCH 11/13] x86/uintr: Introduce uintr_wait() syscall Sohil Mehta
2021-09-24 11:04   ` Thomas Gleixner
2021-09-25 12:08     ` Thomas Gleixner
2021-09-28 23:13       ` Sohil Mehta
2021-09-28 23:08     ` Sohil Mehta
2021-09-26 14:41   ` Thomas Gleixner
2021-09-29  1:09     ` Sohil Mehta
2021-09-29  3:30   ` Andy Lutomirski
2021-09-29  4:56     ` Sohil Mehta
2021-09-30 18:08       ` Andy Lutomirski
2021-09-30 19:29         ` Thomas Gleixner
2021-09-30 22:01           ` Andy Lutomirski
2021-10-01  0:01             ` Thomas Gleixner
2021-10-01  4:41               ` Andy Lutomirski
2021-10-01  9:56                 ` Thomas Gleixner
2021-10-01 15:13                   ` Andy Lutomirski
2021-10-01 18:04                     ` Sohil Mehta
2021-10-01 21:29                     ` Thomas Gleixner
2021-10-01 23:00                       ` Sohil Mehta
2021-10-01 23:04                       ` Andy Lutomirski
2021-09-13 20:01 ` [RFC PATCH 12/13] x86/uintr: Wire up the user interrupt syscalls Sohil Mehta
2021-09-13 20:01 ` [RFC PATCH 13/13] selftests/x86: Add basic tests for User IPI Sohil Mehta
2021-09-13 20:27 ` [RFC PATCH 00/13] x86 User Interrupts support Dave Hansen
2021-09-14 19:03   ` Mehta, Sohil
2021-09-23 12:19     ` Greg KH
2021-09-23 14:09       ` Greg KH
2021-09-23 14:46         ` Dave Hansen
2021-09-23 15:07           ` Greg KH
2021-09-23 23:24         ` Sohil Mehta
2021-09-23 23:09       ` Sohil Mehta
2021-09-24  0:17       ` Sohil Mehta
2021-09-23 14:39 ` Jens Axboe
2021-09-29  4:31 ` Andy Lutomirski
2021-09-30 16:30   ` Stefan Hajnoczi
2021-09-30 17:24     ` Sohil Mehta
2021-09-30 17:26       ` Andy Lutomirski
2021-10-01 16:35       ` Stefan Hajnoczi [this message]
2021-10-01 16:35         ` Stefan Hajnoczi
2021-10-01 16:41         ` Richard Henderson
2021-10-01 16:41           ` Richard Henderson
2021-09-30 16:26 ` Stefan Hajnoczi
2021-10-01  0:40   ` Sohil Mehta
2021-10-01  8:19 ` Pavel Machek
2021-11-18 22:19   ` Sohil Mehta
2021-11-16  3:49 ` Prakash Sangappa
2021-11-18 21:44   ` Sohil Mehta
2021-12-22 16:17 ` Chrisma Pakha
2022-01-07  2:08   ` Sohil Mehta
2022-01-17  1:14     ` Chrisma Pakha

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