Messages in this thread | | | Subject | RE: [PATCH v2:3/3]Export cpu topology by sysfs | Date | Wed, 28 Dec 2005 21:46:11 +0800 | From | "Zhang, Yanmin" <> |
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>>-----Original Message----- >>From: Nathan Lynch [mailto:ntl@pobox.com] >>Sent: 2005年12月28日 5:20 >>To: Yanmin Zhang >>Cc: greg@kroah.com; linux-kernel@vger.kernel.org; discuss@x86-64.org; linux-ia64@vger.kernel.org; Siddha, Suresh B; Shah, Rajesh; >>Pallipadi, Venkatesh; Zhang, Yanmin >>Subject: Re: [PATCH v2:3/3]Export cpu topology by sysfs >> >>Yanmin Zhang wrote: >>> >>> Items (attributes) are similar to /proc/cpuinfo. >>> >>> 1) /sys/devices/system/cpu/cpuX/topology/physical_package_id: >>> represent the physical package id of cpu X; >>> 2) /sys/devices/system/cpu/cpuX/topology/core_id: >>> represent the cpu core id to cpu X; >>> 3) /sys/devices/system/cpu/cpuX/topology/thread_id: >>> represent the cpu thread id to cpu X; >> >>So what is the format of the *_id attributes? Looks like this is >>determined by the architecture, which is okay, but it doesn't seem >>explicit. The type of *_id is int or u8 (like on x86_64). It's better to convert the value to int.
>> >>What about sane default values for the *_id attributes? It depends on the specific architectures. On i386/x86_64, the default vaules of *_id are 0xffu. On ia64, the default value of physical_package_id is -1.
For example, >>say I have a uniprocessor PC without HT or multicore -- should all of >>these attributes have zero values, or some kind of "special" value to >>mean "not applicable"? This feature is disabled when CONFIG_SMP=n. I can't make decision that all arch should use the same default values, so let architectures to decide. Is it ok?
>> >>Hmm, why should thread_id be exported at all? Is it useful to >>userspace in a way that the logical cpu id is not? Just to make it clearer. Of course, physical_package_id /core_id/ logical cpu id could tell enough info like thread id.
>> >>> 4) /sys/devices/system/cpu/cpuX/topology/thread_siblings: >>> represent the thread siblings to cpu X in the same core; >>> 5) /sys/devices/system/cpu/cpuX/topology/core_siblings: >>> represent the thread siblings to cpu X in the same physical package; >>> >>> If one architecture wants to support this feature, it just needs to >>> implement 5 defines, typically in file include/asm-XXX/topology.h. >>> The 5 defines are: >>> #define topology_physical_package_id(cpu) >>> #define topology_core_id(cpu) >>> #define topology_thread_id(cpu) >>> #define topology_thread_siblings(cpu) >>> #define topology_core_siblings(cpu) >>> >>> The type of siblings is cpumask_t. >>> >>> If an attribute isn't defined on an architecture, it won't be >>> exported. >> >>Okay, but which combinations of attributes are valid? E.g. I would >>think that it's fine for an architecture to define topology_thread_id >>and topology_thread_siblings without any of the others, correct? I think topology_physical_package_id/topology_core_id/topology_core_siblings are also needed. For example, admin might want to bind some processes to specific physical cpu, or core.
>> >>Also I'd rather the architectures have the ability to define these as >>functions instead of macros. It's easy. The architectures just need the defines to point to functions in their specific header files, typically in file include/asm-XXX/topology.h.
>> >><snip> >> >>> +/* Add/Remove cpu_topology interface for CPU device */ >>> +static int __cpuinit topology_add_dev(struct sys_device * sys_dev) >>> +{ >>> + sysfs_create_group(&sys_dev->kobj, &topology_attr_group); >>> + return 0; >>> +} >> >>Can't sysfs_create_group fail? It might fail, but it doesn't matter. Later when topology_remove_dev is called, sysfs_remove_group will do nothing because the subdir is not created.
>> >>> + >>> +static int __cpuinit topology_remove_dev(struct sys_device * sys_dev) >>> +{ >>> + sysfs_remove_group(&sys_dev->kobj, &topology_attr_group); >>> + return 0; >>> +} >>> + >>> +static int __cpuinit topology_cpu_callback(struct notifier_block *nfb, >>> + unsigned long action, void *hcpu) >>> +{ >>> + unsigned int cpu = (unsigned long)hcpu; >>> + struct sys_device *sys_dev; >>> + >>> + sys_dev = get_cpu_sysdev(cpu); >>> + switch (action) { >>> + case CPU_ONLINE: >>> + topology_add_dev(sys_dev); >>> + break; >>> + case CPU_DEAD: >>> + topology_remove_dev(sys_dev); >>> + break; >>> + } >>> + return NOTIFY_OK; >>> +} >> >>I don't think it makes much sense to add and remove the attribute >>group for cpu online/offline events. The topology information for an >>offline cpu is potentially valuable -- it could help the admin decide >>which processor to online at runtime, for example. >> >>I believe the correct time to update the topology information is when >>the topology actually changes (e.g. physical addition or removal of a >>processor) -- this is independent of online/offline operations. Currently, on i386/x86_64/ia64, a cpu gets its own topology by itself and fills into a global array. If the cpu is offline since the machine is booted, we can't get its topology info. And when a cpu is offline, current kernel deletes it from the thread_siblings and core_siblings of other cpu.
>> >>In general, I'm still a little uneasy with exporting the cpu topology >>in this way. I can see how the information would be useful, and right >>now, Linux does not do a great job of exposing to userspace these >>relationships between cpus. So I see the need. I also think there are requirements. Many high-reliable applications, such like in telecom, need bind processes on specific cpus. The topology info is important for admin to do so.
But the things about >>this approach which I don't like are: >> >>- Attributes which are not applicable to the running system will be >> exported anyway. Discovery at runtime would be less confusing, I >> think. >> >>- This locks us into exporting a three-level topology (thread, core, >> package), with hard-coded names, when it seems probable that there >> will be systems with more levels than that in the future. It's easy to add more levels based on my implementations. Hard-coded names might be a problem. Is there any special requirement to change the names arch-specifically? If some architectures really need their specific names, I will change the names from hard-coded to arch-defined.
>> >>Have you considered basing the exported topology on sched domains? No. Sched domains consist of far more info. Zou Nanhai wrote a patch to export sched domain info by procfs. I think it's better if we could export sched domain by sysfs.
Thanks, Yanmin - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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