Messages in this thread | | | Date | Wed, 4 Jul 2018 13:39:19 +0530 | From | Gautham R Shenoy <> | Subject | Re: [v2 PATCH 1/2] powerpc: Detect the presence of big-cores via "ibm, thread-groups" |
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Hello Murilo,
Thanks for reviewing the patch. Replies inline.
On Tue, Jul 03, 2018 at 02:16:55PM -0300, Murilo Opsfelder Araujo wrote: > On Tue, Jul 03, 2018 at 04:33:50PM +0530, Gautham R. Shenoy wrote: > > From: "Gautham R. Shenoy" <ego@linux.vnet.ibm.com> > > > > On IBM POWER9, the device tree exposes a property array identifed by > > "ibm,thread-groups" which will indicate which groups of threads share a > > particular set of resources. > > > > As of today we only have one form of grouping identifying the group of > > threads in the core that share the L1 cache, translation cache and > > instruction data flow. > > > > This patch defines the helper function to parse the contents of > > "ibm,thread-groups" and a new structure to contain the parsed output. > > > > The patch also creates the sysfs file named "small_core_siblings" that > > returns the physical ids of the threads in the core that share the L1 > > cache, translation cache and instruction data flow. > > > > Signed-off-by: Gautham R. Shenoy <ego@linux.vnet.ibm.com> > > --- > > Documentation/ABI/testing/sysfs-devices-system-cpu | 8 ++ > > arch/powerpc/include/asm/cputhreads.h | 22 +++++ > > arch/powerpc/kernel/setup-common.c | 110 +++++++++++++++++++++ > > arch/powerpc/kernel/sysfs.c | 35 +++++++ > > 4 files changed, 175 insertions(+) > > > > diff --git a/Documentation/ABI/testing/sysfs-devices-system-cpu b/Documentation/ABI/testing/sysfs-devices-system-cpu > > index 9c5e7732..53a823a 100644 > > --- a/Documentation/ABI/testing/sysfs-devices-system-cpu > > +++ b/Documentation/ABI/testing/sysfs-devices-system-cpu > > @@ -487,3 +487,11 @@ Description: Information about CPU vulnerabilities > > "Not affected" CPU is not affected by the vulnerability > > "Vulnerable" CPU is affected and no mitigation in effect > > "Mitigation: $M" CPU is affected and mitigation $M is in effect > > + > > +What: /sys/devices/system/cpu/cpu[0-9]+/small_core_sibings > > s/small_core_sibings/small_core_siblings
Nice catch! Will fix this. > > By the way, big_core_siblings was mentioned in the introductory email.
It should be small_core_siblings in the introductory e-mail. My bad.
> > > +Date: 03-Jul-2018 > > +KernelVersion: v4.18.0 > > +Contact: Gautham R. Shenoy <ego@linux.vnet.ibm.com> > > +Description: List of Physical ids of CPUs which share the the L1 cache, > > + translation cache and instruction data-flow with this CPU. > > +Values: Comma separated list of decimal integers.
[..snip..]
> > +/* > > + * parse_thread_groups: Parses the "ibm,thread-groups" device tree > > + * property for the CPU device node dn and stores > > + * the parsed output in the thread_groups > > + * structure tg. > > Perhaps document the arguments of this function, as done in the second > patch?
Will do this. Thanks.
> > > + * > > + * ibm,thread-groups[0..N-1] array defines which group of threads in > > + * the CPU-device node can be grouped together based on the property. > > + * > > + * ibm,thread-groups[0] tells us the property based on which the > > + * threads are being grouped together. If this value is 1, it implies > > + * that the threads in the same group share L1, translation cache. > > + * > > + * ibm,thread-groups[1] tells us how many such thread groups exist. > > + * > > + * ibm,thread-groups[2] tells us the number of threads in each such > > + * group. > > + * > > + * ibm,thread-groups[3..N-1] is the list of threads identified by > > + * "ibm,ppc-interrupt-server#s" arranged as per their membership in > > + * the grouping. > > + * > > + * Example: If ibm,thread-groups = [1,2,4,5,6,7,8,9,10,11,12] it > > + * implies that there are 2 groups of 4 threads each, where each group > > + * of threads share L1, translation cache. > > + * > > + * The "ibm,ppc-interrupt-server#s" of the first group is {5,6,7,8} > > + * and the "ibm,ppc-interrupt-server#s" of the second group is {9, 10, > > + * 11, 12} structure > > + * > > + * Returns 0 on success, -EINVAL if the property does not exist, > > + * -ENODATA if property does not have a value, and -EOVERFLOW if the > > + * property data isn't large enough. > > + */ > > +int parse_thread_groups(struct device_node *dn, > > + struct thread_groups *tg) > > +{ > > + unsigned int nr_groups, threads_per_group, property; > > + int i; > > + u32 thread_group_array[3 + MAX_THREAD_LIST_SIZE]; > > + u32 *thread_list; > > + size_t total_threads; > > + int ret; > > + > > + ret = of_property_read_u32_array(dn, "ibm,thread-groups", > > + thread_group_array, 3); > > + > > + if (ret) > > + return ret; > > + > > + property = thread_group_array[0]; > > + nr_groups = thread_group_array[1]; > > + threads_per_group = thread_group_array[2]; > > + total_threads = nr_groups * threads_per_group; > > + > > + ret = of_property_read_u32_array(dn, "ibm,thread-groups", > > + thread_group_array, > > + 3 + total_threads); > > + if (ret) > > + return ret; > > + > > + thread_list = &thread_group_array[3]; > > + > > + for (i = 0 ; i < total_threads; i++) > > + tg->thread_list[i] = thread_list[i]; > > + > > + tg->property = property; > > + tg->nr_groups = nr_groups; > > + tg->threads_per_group = threads_per_group; > > + > > + return 0; > > +} > > + > > +/* > > + * get_cpu_thread_group_start : Searches the thread group in tg->thread_list > > + * that @cpu belongs to. > > Same here.
Sure. > > > + * > > + * Returns the index to tg->thread_list that points to the the start > > + * of the thread_group that @cpu belongs to. > > + * > > + * Returns -1 if cpu doesn't belong to any of the groups pointed > > + * to by tg->thread_list. > > + */ > > +int get_cpu_thread_group_start(int cpu, struct thread_groups *tg) > > +{ > > + int hw_cpu_id = get_hard_smp_processor_id(cpu); > > + int i, j; > > + > > + for (i = 0; i < tg->nr_groups; i++) { > > + int group_start = i * tg->threads_per_group; > > + > > + for (j = 0; j < tg->threads_per_group; j++) { > > + int idx = group_start + j; > > + > > + if (tg->thread_list[idx] == hw_cpu_id) > > + return group_start; > > + } > > + } > > + > > + return -1; > > +} > > + > > /** > > * setup_cpu_maps - initialize the following cpu maps: > > * cpu_possible_mask > > @@ -467,6 +571,7 @@ void __init smp_setup_cpu_maps(void) > > const __be32 *intserv; > > __be32 cpu_be; > > int j, len; > > + struct thread_groups tg = {.nr_groups = 0}; > > We assume has_big_cores = true but here we initialize .nr_groups > otherwise. It's kind of contradictory.
.nr_groups is being initialized to some sane value here. Perhaps I should move the initializations of tg.nr_groups and tg.property inside parse_thread_groups.
> > What if has_big_cores is assumed false and members of tg are initialized > with zeroes?
The idea here is that after parsing all the CPU nodes, the variable "has_big_cores" continues to remain to true if all the CPU nodes are big cores. Even if one of them isn't a big core (not sure if this is possible in practise) then we want to set it to false.
Hence we start with the assumption that has_big_cores is true, and switch it on finding even one core that is not a big-core.
But I got to know that this is an overkill since if the component small core is bad, the entire big-core is disabled. Thus it might be sufficient to just check for one CPU node, if it is a big core or not, and set the variable from "false" to "true".
> > > > > DBG(" * %pOF...\n", dn); > > > > @@ -505,6 +610,11 @@ void __init smp_setup_cpu_maps(void) > > cpu++; > > } > > > > + if (parse_thread_groups(dn, &tg) || > > + tg.nr_groups < 1 || tg.property != 1) { > > + has_big_cores = false; > > + } > > + > > parse_thread_groups() returns before setting tg.property if property > doesn't exist. Are we confident that tg.property won't contain any > garbage that could lead to a false positive here? Shouldn't we also > initialize .property when declaring tg?
Yes we should. Will move the initializations to parse_thread_groups.
> > What if this logic is encapsulated in a function? For example: > > has_big_cores = dt_has_big_cores(dn, &tg);
Good idea.
> > > if (cpu >= nr_cpu_ids) { > > of_node_put(dn); > > break; > > diff --git a/arch/powerpc/kernel/sysfs.c b/arch/powerpc/kernel/sysfs.c [..snip..]
Will address these changes in the subsequent patch series. > > Cheers > Murilo
-- Thanks and Regards gautham.
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