Re: [Qemu-devel] [Qemu-ppc] [RFC PATCH] smp: autodetect numbers of threads per core

[Alexey Kardashevskiy <aik@ozlabs.ru>]

On 12/04/2013 04:34 PM, Alexey Kardashevskiy wrote:
> On 11/16/2013 03:58 AM, Alexey Kardashevskiy wrote:
>> On 16.11.2013 0:15, Alexander Graf wrote:
>>>
>>>
>>> Am 15.11.2013 um 00:12 schrieb Alexey Kardashevskiy <aik@ozlabs.ru>:
>>>
>>>> At the moment only a whole CPU core can be assigned to a KVM. Since
>>>> POWER7/8 support several threads per core, we want all threads of a core
>>>> to go to the same KVM so every time we run QEMU with -enable-kvm on
>>>> POWER, we have to add -smp X,threads=(4|8)" (4 for POWER7 and
>>>> 8 for POWER8).
>>>>
>>>> This patch tries to read smp_threads number from an accelerator and
>>>> falls back to the default value (1) if the accelerator did not care
>>>> to change it.
>>>>
>>>> Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
>>>> ---
>>>>
>>>> (!!!)
>>>>
>>>> The usual question - what would be the normal way of doing this?
>>>> What does this patch break? I cannot think of anything right now.
>>>
>>> Is this really what the user wants? On p7 you can run in no-smt, smt2
>>> and smt4 mode. Today we simply default to no-smt. Changing defaults
>>> is usually a bad thing.
>>
>>
>> Defaulting to 1 thread on P7 is a bad thing (other threads stay unused -
>> what is good about this?) and the only reason which I know why it is
>> still threads=1 is that it is hard to make a patch for upstream to
>> change this default.
>>
>>
>> Paul, please, help.
> 
> 
> Anyone, ping? Thanks.


Everyone ignores me :(


> 
> 
>>> Alex
>>>
>>>>
>>>>
>>>> ---
>>>> target-ppc/kvm.c |  6 ++++++
>>>> vl.c             | 29 ++++++++++++++++-------------
>>>> 2 files changed, 22 insertions(+), 13 deletions(-)
>>>>
>>>> diff --git a/target-ppc/kvm.c b/target-ppc/kvm.c
>>>> index 10d0cd9..80c0386 100644
>>>> --- a/target-ppc/kvm.c
>>>> +++ b/target-ppc/kvm.c
>>>> @@ -109,6 +109,12 @@ int kvm_arch_init(KVMState *s)
>>>>                         "VM to stall at times!\n");
>>>>     }
>>>>
>>>> +    if (!smp_threads) {
>>>> +        smp_threads = cap_ppc_smt;
>>>> +    } else {
>>>> +        smp_threads = MIN(smp_threads, cap_ppc_smt);
>>>> +    }
>>>> +
>>>>     kvm_ppc_register_host_cpu_type();
>>>>
>>>>     return 0;
>>>> diff --git a/vl.c b/vl.c
>>>> index 4ad15b8..97fa203 100644
>>>> --- a/vl.c
>>>> +++ b/vl.c
>>>> @@ -210,7 +210,7 @@ int singlestep = 0;
>>>> int smp_cpus = 1;
>>>> int max_cpus = 0;
>>>> int smp_cores = 1;
>>>> -int smp_threads = 1;
>>>> +int smp_threads = 0;
>>>> #ifdef CONFIG_VNC
>>>> const char *vnc_display;
>>>> #endif
>>>> @@ -1395,7 +1395,9 @@ static void smp_parse(QemuOpts *opts)
>>>>         if (cpus == 0 || sockets == 0) {
>>>>             sockets = sockets > 0 ? sockets : 1;
>>>>             cores = cores > 0 ? cores : 1;
>>>> -            threads = threads > 0 ? threads : 1;
>>>> +            if (!threads) {
>>>> +                threads = smp_threads > 0 ? smp_threads : 1;
>>>> +            }
>>>>             if (cpus == 0) {
>>>>                 cpus = cores * threads * sockets;
>>>>             }
>>>> @@ -1413,7 +1415,8 @@ static void smp_parse(QemuOpts *opts)
>>>>         smp_cpus = cpus;
>>>>         smp_cores = cores > 0 ? cores : 1;
>>>>         smp_threads = threads > 0 ? threads : 1;
>>>> -
>>>> +    } else if (!smp_threads) {
>>>> +        smp_threads = 1;
>>>>     }
>>>>
>>>>     if (max_cpus == 0) {
>>>> @@ -3880,16 +3883,6 @@ int main(int argc, char **argv, char **envp)
>>>>         data_dir[data_dir_idx++] = CONFIG_QEMU_DATADIR;
>>>>     }
>>>>
>>>> -    smp_parse(qemu_opts_find(qemu_find_opts("smp-opts"), NULL));
>>>> -
>>>> -    machine->max_cpus = machine->max_cpus ?: 1; /* Default to UP */
>>>> -    if (smp_cpus > machine->max_cpus) {
>>>> -        fprintf(stderr, "Number of SMP cpus requested (%d), exceeds max cpus "
>>>> -                "supported by machine `%s' (%d)\n", smp_cpus,  machine->name,
>>>> -                machine->max_cpus);
>>>> -        exit(1);
>>>> -    }
>>>> -
>>>>     /*
>>>>      * Get the default machine options from the machine if it is not already
>>>>      * specified either by the configuration file or by the command line.
>>>> @@ -4039,6 +4032,16 @@ int main(int argc, char **argv, char **envp)
>>>>
>>>>     configure_accelerator();
>>>>
>>>> +    smp_parse(qemu_opts_find(qemu_find_opts("smp-opts"), NULL));
>>>> +
>>>> +    machine->max_cpus = machine->max_cpus ?: 1; /* Default to UP */
>>>> +    if (smp_cpus > machine->max_cpus) {
>>>> +        fprintf(stderr, "Number of SMP cpus requested (%d), exceeds max cpus "
>>>> +                "supported by machine `%s' (%d)\n", smp_cpus,  machine->name,
>>>> +                machine->max_cpus);
>>>> +        exit(1);
>>>> +    }
>>>> +
>>>>     if (!qtest_enabled() && qtest_chrdev) {
>>>>         qtest_init();
>>>>     }
>>
>>
> 
> 


-- 
Alexey