Messages in this thread | | | Date | Wed, 12 Sep 2018 16:56:19 +0100 | From | Patrick Bellasi <> | Subject | Re: [PATCH v4 02/16] sched/core: uclamp: map TASK's clamp values into CPU's clamp groups |
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On 12-Sep 15:49, Peter Zijlstra wrote: > On Tue, Aug 28, 2018 at 02:53:10PM +0100, Patrick Bellasi wrote: > > +/** > > + * Utilization's clamp group > > + * > > + * A utilization clamp group maps a "clamp value" (value), i.e. > > + * util_{min,max}, to a "clamp group index" (group_id). > > + */ > > +struct uclamp_se { > > + unsigned int value; > > + unsigned int group_id; > > +}; > > > +/** > > + * uclamp_map: reference counts a utilization "clamp value" > > + * @value: the utilization "clamp value" required > > + * @se_count: the number of scheduling entities requiring the "clamp value" > > + * @se_lock: serialize reference count updates by protecting se_count > > Why do you have a spinlock to serialize a single value? Don't we have > atomics for that?
There are some code paths where it's used to protect clamp groups mapping and initialization, e.g.
uclamp_group_get() spin_lock() // initialize clamp group (if required) and then... se_count += 1 spin_unlock()
Almost all these paths are triggered from user-space and protected by a global uclamp_mutex, but fork/exit paths.
To serialize these paths I'm using the spinlock above, does it make sense ? Can we use the global uclamp_mutex on forks/exit too ?
One additional observations is that, if in the future we want to add a kernel space API, (e.g. driver asking for a new clamp value), maybe we will need to have a serialized non-sleeping uclamp_group_get() API ?
> > + */ > > +struct uclamp_map { > > + int value; > > + int se_count; > > + raw_spinlock_t se_lock; > > +}; > > + > > +/** > > + * uclamp_maps: maps each SEs "clamp value" into a CPUs "clamp group" > > + * > > + * Since only a limited number of different "clamp values" are supported, we > > + * need to map each different clamp value into a "clamp group" (group_id) to > > + * be used by the per-CPU accounting in the fast-path, when tasks are > > + * enqueued and dequeued. > > + * We also support different kind of utilization clamping, min and max > > + * utilization for example, each representing what we call a "clamp index" > > + * (clamp_id). > > + * > > + * A matrix is thus required to map "clamp values" to "clamp groups" > > + * (group_id), for each "clamp index" (clamp_id), where: > > + * - rows are indexed by clamp_id and they collect the clamp groups for a > > + * given clamp index > > + * - columns are indexed by group_id and they collect the clamp values which > > + * maps to that clamp group > > + * > > + * Thus, the column index of a given (clamp_id, value) pair represents the > > + * clamp group (group_id) used by the fast-path's per-CPU accounting. > > + * > > + * NOTE: first clamp group (group_id=0) is reserved for tracking of non > > + * clamped tasks. Thus we allocate one more slot than the value of > > + * CONFIG_UCLAMP_GROUPS_COUNT. > > + * > > + * Here is the map layout and, right below, how entries are accessed by the > > + * following code. > > + * > > + * uclamp_maps is a matrix of > > + * +------- UCLAMP_CNT by CONFIG_UCLAMP_GROUPS_COUNT+1 entries > > + * | | > > + * | /---------------+---------------\ > > + * | +------------+ +------------+ > > + * | / UCLAMP_MIN | value | | value | > > + * | | | se_count |...... | se_count | > > + * | | +------------+ +------------+ > > + * +--+ +------------+ +------------+ > > + * | | value | | value | > > + * \ UCLAMP_MAX | se_count |...... | se_count | > > + * +-----^------+ +----^-------+ > > + * | | > > + * uc_map = + | > > + * &uclamp_maps[clamp_id][0] + > > + * clamp_value = > > + * uc_map[group_id].value > > + */ > > +static struct uclamp_map uclamp_maps[UCLAMP_CNT] > > + [CONFIG_UCLAMP_GROUPS_COUNT + 1] > > + ____cacheline_aligned_in_smp; > > + > > I'm still completely confused by all this. > > sizeof(uclamp_map) = 12 > > that array is 2*6=12 of those, so the whole thing is 144 bytes. which is > more than 2 (64 byte) cachelines.
This data structure is *not* used in the hot-path, that's why I did not care about fitting it exactly into few cache lines.
It's used to map a user-space "clamp value" into a kernel-space "clamp group" when user-space: - changes a task specific clamp value - changes a cgroup clamp value - a task forks/exits
I assume we can consider all those as "slow" code paths, is that correct ?
At enqueue/dequeue time we use instead struct uclamp_cpu, introduced by the next patch:
[PATCH v4 03/16] sched/core: uclamp: add CPU's clamp groups accounting https://lore.kernel.org/lkml/20180828135324.21976-4-patrick.bellasi@arm.com/
That's where we refcount RUNNABLE tasks and we have to figure out the current clamp value for a CPU.
That data structure, with CONFIG_UCLAMP_GROUPS_COUNT=5, is:
struct uclamp_cpu { struct uclamp_group group[2][6]; /* 0 96 */ /* --- cacheline 1 boundary (64 bytes) was 32 bytes ago --- */ int value[2]; /* 96 8 */ int flags; /* 104 4 */
/* size: 108, cachelines: 2, members: 3 */ /* last cacheline: 44 bytes */ };
and we fit into 2 cache lines with this data layout:
util_min[0..5] | util_max[0..5] | other data
> What's the purpose of that cacheline align statement?
In uclamp_maps, we still need to scan the array when a clamp value is changed from user-space, i.e. the cases reported above. Thus, that alignment is just to ensure that we minimize the number of cache lines used. Does that make sense ?
Maybe that alignment implicitly generated by the compiler ?
> Note that without that apparently superfluous lock, it would be 8*12 = > 96 bytes, which is 1.5 lines and would indeed suggest you default to > GROUP_COUNT=7 by default to fill 2 lines.
Yes, will check better if we can count on just the uclamp_mutex
> Why are the min and max things torn up like that? I'm fairly sure I > asked some of that last time; but the above comments only try to explain > what, not why.
We use that organization to speedup scanning for clamp values of the same clamp_id. That's more important in the hot-path than above, where we need to scan struct uclamp_cpu when a new aggregated clamp value has to be computed. This is done in:
[PATCH v4 03/16] sched/core: uclamp: add CPU's clamp groups accounting https://lore.kernel.org/lkml/20180828135324.21976-4-patrick.bellasi@arm.com/
Specifically:
dequeue_task() uclamp_cpu_put() uclamp_cpu_put_id(clamp_id) uclamp_cpu_update(clamp_id) // Here we have an array scan by clamp_id
With the given data layout I reported above, when we update the min_clamp value (boost) we have all the data required in a single cache line.
If that makes sense, I can certainly improve the comment above to justify its layout.
Cheers, Patrick
-- #include <best/regards.h>
Patrick Bellasi
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