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

[Alexey Kardashevskiy]

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 <address@hidden>:
>>
>>> 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 <address@hidden>
>>> ---
>>>
>>> (!!!)
>>>
>>> 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.


>> 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