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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: Mon, 6 May 2019 11:23:11 +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: Mon, 06 May 2019 09:23:27 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
  • Openpgp: preference=signencrypt

On 06/05/2019 10:57, Jan Beulich wrote:
>>>> On 06.05.19 at 08:56, <jgross@xxxxxxxx> wrote:
>> --- a/xen/arch/x86/setup.c
>> +++ b/xen/arch/x86/setup.c
>> @@ -1701,6 +1701,8 @@ void __init noreturn __start_xen(unsigned long mbi_p)
>>          printk(XENLOG_INFO "Parked %u CPUs\n", num_parked);
>>      smp_cpus_done();
>>  
>> +    scheduler_smp_init();
>> +
>>      do_initcalls();
> 
> This placement and the actual implementation of the function make
> me wonder: Why didn't you make this an initcall, thus taking care of
> Arm (at least in an abstract way) at the same time?

Hmm, true. Will change.

> 
>>  void scheduler_percpu_init(unsigned int cpu)
>>  {
>>      struct scheduler *sched = per_cpu(scheduler, cpu);
>>      struct sched_resource *sd = per_cpu(sched_res, cpu);
>> +    const cpumask_t *mask;
>> +    unsigned int master_cpu;
>> +    spinlock_t *lock;
>> +    struct sched_item *old_item, *master_item;
>> +
>> +    if ( system_state == SYS_STATE_resume )
>> +        return;
>> +
>> +    switch ( opt_sched_granularity )
>> +    {
>> +    case SCHED_GRAN_cpu:
>> +        mask = cpumask_of(cpu);
>> +        break;
>> +    case SCHED_GRAN_core:
>> +        mask = per_cpu(cpu_sibling_mask, cpu);
>> +        break;
>> +    case SCHED_GRAN_socket:
>> +        mask = per_cpu(cpu_core_mask, cpu);
>> +        break;
>> +    default:
>> +        ASSERT_UNREACHABLE();
>> +        return;
>> +    }
>>  
>> -    if ( system_state != SYS_STATE_resume )
>> +    if ( cpu == 0 || cpumask_weight(mask) == 1 )
> 
> At least outside of x86 specific code I think we should avoid
> introducing (further?) assumptions that seeing CPU 0 on a
> CPU initialization path implies this being while booting the
> system. I wonder anyway whether the right side of the ||
> doesn't render the left side redundant.

On the boot cpu this function is called before e.g. cpu_sibling_mask
is initialized. I can have a try using:

if ( cpumask_weight(mask) <= 1 )

> 
>> +static unsigned int __init sched_check_granularity(void)
>> +{
>> +    unsigned int cpu;
>> +    unsigned int siblings, gran = 0;
>> +
>> +    for_each_online_cpu( cpu )
> 
> You want to decide for one of two possible styles, but not a mixture
> of both:
> 
>     for_each_online_cpu ( cpu )
> 
> or
> 
>     for_each_online_cpu(cpu)

Sorry, will correct.

> 
> . Yet then I'm a little puzzled by its use here in the first place.
> Generally I think for_each_cpu() uses in __init functions are
> problematic, as they then require further code elsewhere to
> deal with hot-onlining. A pre-SMP-initcall plus use of CPU
> notifiers is typically more appropriate.

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.


Juergen

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