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Re: [Xen-devel] [PATCH RFC V2 45/45] xen/sched: add scheduling granularity enum

  • To: Jan Beulich <JBeulich@xxxxxxxx>
  • From: Juergen Gross <jgross@xxxxxxxx>
  • Date: Fri, 10 May 2019 11:00:39 +0200
  • Autocrypt: addr=jgross@xxxxxxxx; prefer-encrypt=mutual; keydata= mQENBFOMcBYBCACgGjqjoGvbEouQZw/ToiBg9W98AlM2QHV+iNHsEs7kxWhKMjrioyspZKOB ycWxw3ie3j9uvg9EOB3aN4xiTv4qbnGiTr3oJhkB1gsb6ToJQZ8uxGq2kaV2KL9650I1SJve dYm8Of8Zd621lSmoKOwlNClALZNew72NjJLEzTalU1OdT7/i1TXkH09XSSI8mEQ/ouNcMvIJ NwQpd369y9bfIhWUiVXEK7MlRgUG6MvIj6Y3Am/BBLUVbDa4+gmzDC9ezlZkTZG2t14zWPvx XP3FAp2pkW0xqG7/377qptDmrk42GlSKN4z76ELnLxussxc7I2hx18NUcbP8+uty4bMxABEB AAG0H0p1ZXJnZW4gR3Jvc3MgPGpncm9zc0BzdXNlLmNvbT6JATkEEwECACMFAlOMcK8CGwMH CwkIBwMCAQYVCAIJCgsEFgIDAQIeAQIXgAAKCRCw3p3WKL8TL8eZB/9G0juS/kDY9LhEXseh mE9U+iA1VsLhgDqVbsOtZ/S14LRFHczNd/Lqkn7souCSoyWsBs3/wO+OjPvxf7m+Ef+sMtr0 G5lCWEWa9wa0IXx5HRPW/ScL+e4AVUbL7rurYMfwCzco+7TfjhMEOkC+va5gzi1KrErgNRHH kg3PhlnRY0Udyqx++UYkAsN4TQuEhNN32MvN0Np3WlBJOgKcuXpIElmMM5f1BBzJSKBkW0Jc Wy3h2Wy912vHKpPV/Xv7ZwVJ27v7KcuZcErtptDevAljxJtE7aJG6WiBzm+v9EswyWxwMCIO RoVBYuiocc51872tRGywc03xaQydB+9R7BHPuQENBFOMcBYBCADLMfoA44MwGOB9YT1V4KCy vAfd7E0BTfaAurbG+Olacciz3yd09QOmejFZC6AnoykydyvTFLAWYcSCdISMr88COmmCbJzn sHAogjexXiif6ANUUlHpjxlHCCcELmZUzomNDnEOTxZFeWMTFF9Rf2k2F0Tl4E5kmsNGgtSa aMO0rNZoOEiD/7UfPP3dfh8JCQ1VtUUsQtT1sxos8Eb/HmriJhnaTZ7Hp3jtgTVkV0ybpgFg w6WMaRkrBh17mV0z2ajjmabB7SJxcouSkR0hcpNl4oM74d2/VqoW4BxxxOD1FcNCObCELfIS auZx+XT6s+CE7Qi/c44ibBMR7hyjdzWbABEBAAGJAR8EGAECAAkFAlOMcBYCGwwACgkQsN6d 1ii/Ey9D+Af/WFr3q+bg/8v5tCknCtn92d5lyYTBNt7xgWzDZX8G6/pngzKyWfedArllp0Pn fgIXtMNV+3t8Li1Tg843EXkP7+2+CQ98MB8XvvPLYAfW8nNDV85TyVgWlldNcgdv7nn1Sq8g HwB2BHdIAkYce3hEoDQXt/mKlgEGsLpzJcnLKimtPXQQy9TxUaLBe9PInPd+Ohix0XOlY+Uk QFEx50Ki3rSDl2Zt2tnkNYKUCvTJq7jvOlaPd6d/W0tZqpyy7KVay+K4aMobDsodB3dvEAs6 ScCnh03dDAFgIq5nsB11j3KPKdVoPlfucX2c7kGNH+LUMbzqV6beIENfNexkOfxHf4kBrQQY AQgAIBYhBIUSZ3Lo9gSUpdCX97DendYovxMvBQJa3fDQAhsCAIEJELDendYovxMvdiAEGRYI AB0WIQRTLbB6QfY48x44uB6AXGG7T9hjvgUCWt3w0AAKCRCAXGG7T9hjvk2LAP99B/9FenK/ 1lfifxQmsoOrjbZtzCS6OKxPqOLHaY47BgEAqKKn36YAPpbk09d2GTVetoQJwiylx/Z9/mQI CUbQMg1pNQf9EjA1bNcMbnzJCgt0P9Q9wWCLwZa01SnQWFz8Z4HEaKldie+5bHBL5CzVBrLv 81tqX+/j95llpazzCXZW2sdNL3r8gXqrajSox7LR2rYDGdltAhQuISd2BHrbkQVEWD4hs7iV 1KQHe2uwXbKlguKPhk5ubZxqwsg/uIHw0qZDk+d0vxjTtO2JD5Jv/CeDgaBX4Emgp0NYs8IC UIyKXBtnzwiNv4cX9qKlz2Gyq9b+GdcLYZqMlIBjdCz0yJvgeb3WPNsCOanvbjelDhskx9gd 6YUUFFqgsLtrKpCNyy203a58g2WosU9k9H+LcheS37Ph2vMVTISMszW9W8gyORSgmw==
  • Cc: Tim Deegan <tim@xxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Wei Liu <wei.liu2@xxxxxxxxxx>, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>, George Dunlap <George.Dunlap@xxxxxxxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>, Dario Faggioli <dfaggioli@xxxxxxxx>, Julien Grall <julien.grall@xxxxxxx>, xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Roger Pau Monne <roger.pau@xxxxxxxxxx>
  • Delivery-date: Fri, 10 May 2019 09:00:51 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
  • Openpgp: preference=signencrypt

On 10/05/2019 10:53, Jan Beulich wrote:
>>>> On 08.05.19 at 16:36, <jgross@xxxxxxxx> wrote:
>> On 06/05/2019 12:01, Jan Beulich wrote:
>>>>>> On 06.05.19 at 11:23, <jgross@xxxxxxxx> wrote:
>>>> And that was mentioned in the cover letter: cpu hotplug is not yet
>>>> handled (hence the RFC status of the series).
>>>> When cpu hotplug is being added it might be appropriate to switch the
>>>> scheme as you suggested. Right now the current solution is much more
>>>> simple.
>>> I see (I did notice the cover letter remark, but managed to not
>>> honor it when writing the reply), but I'm unconvinced if incurring
>>> more code churn by not dealing with things the "dynamic" way
>>> right away is indeed the "more simple" (overall) solution.
>> I have started to address cpu on/offlining now.
>> There are multiple design decisions to take.
>> 1. Interaction between sched-gran and smt boot parameters
>> 2. Interaction between sched-gran and xen-hptool smt switching
>> 3. Interaction between sched-gran and single cpu on/offlining
>> Right now any guest won't see a difference regarding sched-gran
>> selection. This means we don't have to think about potential migration
>> restrictions. This might change in future when we want to enable the
>> guest to e.g. use core scheduling themselves in order to mitigate
>> against side channel attacks within the guest.
>> The most simple solution would be (and I'd like to send out V1 of my
>> series with that implemented):
>> sched-gran=core and sched-gran=socket don't allow dynamical switching
>> of smt via xen-hptool.
>> With sched-gran=core or sched-gran=socket offlining a single cpu results
>> in moving the complete core or socket to cpupool_free_cpus and then
>> offlining from there. Only complete cores/sockets can be moved to any
>> cpupool. When onlining a cpu it is added to cpupool_free_cpus and if
>> the core/socket is completely online it will automatically be added to
>> Pool-0 (as today any single onlined cpu).
> Well, this is in line with what was discussed on the call yesterday, so
> I think it's an acceptable initial state to end up in. Albeit, just for
> completeness, I'm not convinced there's no use for "smt-{dis,en}able"
> anymore with core-aware scheduling implemented just in Xen - it
> may still be considered useful as long as we don't expose proper
> topology to guests, for them to be able to do something similar.

As the extra complexity for supporting that is significant I'd like to
at least postpone it. And with the (later) introduction of per-cpupool
smt on/off I guess this would be even less important.


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