lists.openwall.net   lists  /  announce  owl-users  owl-dev  john-users  john-dev  passwdqc-users  yescrypt  popa3d-users  /  oss-security  kernel-hardening  musl  sabotage  tlsify  passwords  /  crypt-dev  xvendor  /  Bugtraq  Full-Disclosure  linux-kernel  linux-netdev  linux-ext4  linux-hardening  linux-cve-announce  PHC 
Open Source and information security mailing list archives
 
Hash Suite: Windows password security audit tool. GUI, reports in PDF.
[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <20231005120557.GA743@noisy.programming.kicks-ass.net>
Date:   Thu, 5 Oct 2023 14:05:57 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Youssef Esmat <youssefesmat@...omium.org>
Cc:     Daniel Jordan <daniel.m.jordan@...cle.com>, mingo@...nel.org,
        vincent.guittot@...aro.org, linux-kernel@...r.kernel.org,
        juri.lelli@...hat.com, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        bristot@...hat.com, corbet@....net, qyousef@...alina.io,
        chris.hyser@...cle.com, patrick.bellasi@...bug.net, pjt@...gle.com,
        pavel@....cz, qperret@...gle.com, tim.c.chen@...ux.intel.com,
        joshdon@...gle.com, timj@....org, kprateek.nayak@....com,
        yu.c.chen@...el.com, joel@...lfernandes.org, efault@....de,
        tglx@...utronix.de
Subject: Re: [PATCH 00/15] sched: EEVDF and latency-nice and/or slice-attr

On Mon, Oct 02, 2023 at 08:41:36PM +0200, Peter Zijlstra wrote:

> When mixing request sizes things become a little more interesting.
> 
> Let me ponder this a little bit more.

Using the attached program (I got *REALLY* fed up trying to draw these
diagrams by hand), let us illustrate the difference between Earliest
*Eligible* Virtual Deadline First and the one with the Eligible test
taken out: EVDF.

Specifically, the program was used with the following argument for
EEVDF:

  ./eevdf -e "0,1024,6" -e "1,1024,2" -e "2,1024,18" -v 19 

and with an additional '-n' for the EVDF column.


EEVDF							EVDF


d = 6							d = 6
d = 2                                                   d = 2
d = 18                                                  d = 18
q = 2                                                   q = 2
                                                        
t=0 V=1                                                 t=0 V=1
 A |----<                                                A |----<
>B  |<                                                  >B  |<
 C   |----------------<                                  C   |----------------<
   |*--------|---------|---------|---------|----           |*--------|---------|---------|---------|----
                                                        
                                                        
t=2 V=1                                                 t=2 V=1
>A |----<                                                A |----<
 B    |<                                                >B    |<
 C   |----------------<                                  C   |----------------<
   |*--------|---------|---------|---------|----           |*--------|---------|---------|---------|----
                                                        
                                                        
t=8 V=3                                                 t=4 V=2
 A       |----<                                         >A |----<
>B    |<                                                 B      |<
 C   |----------------<                                  C   |----------------<
   |--*------|---------|---------|---------|----           |-*-------|---------|---------|---------|----
                                                        
                                                        
t=10 V=4                                                t=10 V=4
 A       |----<                                          A       |----<
 B      |<                                              >B      |<
>C   |----------------<                                  C   |----------------<
   |---*-----|---------|---------|---------|----           |---*-----|---------|---------|---------|----
                                                        
                                                        
t=28 V=10                                               t=12 V=5
 A       |----<                                          A       |----<
>B      |<                                              >B        |<
 C                     |----------------<                C   |----------------<
   |---------*---------|---------|---------|----           |----*----|---------|---------|---------|----
                                                        
                                                        
t=30 V=11                                               t=14 V=5
 A       |----<                                          A       |----<
>B        |<                                            >B          |<
 C                     |----------------<                C   |----------------<
   |---------|*--------|---------|---------|----           |----*----|---------|---------|---------|----
                                                        
                                                        
t=32 V=11                                               t=16 V=6
 A       |----<                                         >A       |----<
>B          |<                                           B            |<
 C                     |----------------<                C   |----------------<
   |---------|*--------|---------|---------|----           |-----*---|---------|---------|---------|----
                                                        
                                                        
t=34 V=12                                               t=22 V=8
>A       |----<                                          A             |----<
 B            |<                                        >B            |<
 C                     |----------------<                C   |----------------<
   |---------|-*-------|---------|---------|----           |-------*-|---------|---------|---------|----
                                                        
                                                        
t=40 V=14                                               t=24 V=9
 A             |----<                                    A             |----<
>B            |<                                        >B              |<
 C                     |----------------<                C   |----------------<
   |---------|---*-----|---------|---------|----           |--------*|---------|---------|---------|----
                                                        
                                                        
t=42 V=15                                               t=26 V=9
 A             |----<                                    A             |----<
>B              |<                                      >B                |<
 C                     |----------------<                C   |----------------<
   |---------|----*----|---------|---------|----           |--------*|---------|---------|---------|----
                                                        
                                                        
t=44 V=15                                               t=28 V=10
 A             |----<                                   >A             |----<
>B                |<                                     B                  |<
 C                     |----------------<                C   |----------------<
   |---------|----*----|---------|---------|----           |---------*---------|---------|---------|----
                                                        
                                                        
t=46 V=16                                               t=34 V=12
>A             |----<                                    A                   |----<
 B                  |<                                  >B                  |<
 C                     |----------------<                C   |----------------<
   |---------|-----*---|---------|---------|----           |---------|-*-------|---------|---------|----
                                                        
                                                        
t=52 V=18                                               t=36 V=13
 A                   |----<                              A                   |----<
>B                  |<                                   B                    |<
 C                     |----------------<               >C   |----------------<
   |---------|-------*-|---------|---------|----           |---------|--*------|---------|---------|----
                                                        
                                                        
t=54 V=19                                               t=54 V=19
 A                   |----<                              A                   |----<
>B                    |<                                >B                    |<
 C                     |----------------<                C                     |----------------<
   |---------|--------*|---------|---------|----           |---------|--------*|---------|---------|----
                                                        
                                                        
lags: -10, 6                                            lags: -7, 11
                                                        
BAaaBCccccccccBBBAaaBBBAaaBB                            BBAaaBBBAaaBBBAaaBCccccccccB



As I wrote before; EVDF has worse lag bounds, but this is not
insurmountable. The biggest problem that I can see is that of wakeup
preemption. Currently we allow to preempt when 'current' has reached V
(RUN_TO_PARITY in pick_eevdf()).

With these rules, when EEVDF schedules C (our large slice task) at t=10
above, it is only a little behind C and can be reaily preempted after
about 2 time units.

However, EVDF will delay scheduling C until much later, see how A and B
walk far ahead of V until t=36. Only when will we pick C. But this means
that we're firmly stuck with C for at least 11 time units. A newly
placed task will be around V and will have no chance to preempt.

That said, I do have me a patch to cure some of that:

  https://git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git/commit/?h=sched/eevdf&id=d7edbe431f31762e516f2730196f41322edcc621

That would allow a task with a shorter request time to preempt in spite
of RUN_TO_PARITY.

However, in this example V is only 2/3 of the way to C's deadline, but
it we were to have many more tasks, you'll see V gets closer and closer
to C's deadline and it will become harder and harder to place such that
preemption becomes viable.

Adding 4 more tasks:

  ./eevdf -e "0,1024,6" -e "1,1024,2" -e "2,1024,18" -v 19 -n -e "3,1024,2" -e "4,1024,2" -e "5,1024,2" -e "6,1024,2"

t=92 V=16
 A                   |----<
 B                    |<
>C   |----------------<
 D                    |<
 E                   |<
 F                    |<
 G                   |<
   |---------|-----*---|---------|---------|----


And I worry this will create very real latency spikes.

That said; I do see not having the eligibility check can help. So I'm
not opposed to having a sched_feat for this, but I would not want to
default to EVDF.

View attachment "eevdf.c" of type "text/x-csrc" (4010 bytes)

Powered by blists - more mailing lists

Powered by Openwall GNU/*/Linux Powered by OpenVZ