Re: [this_cpu_xx V7 0/8] Per cpu atomics in core allocators and cleanup

[Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>]

* Christoph Lameter (cl@linux-foundation.org) wrote:
> Leftovers from the earlier patchset. Mostly applications of per cpu counters
> to core components.
> 
> After this patchset there will be only one user of local_t left: Mathieu's
> trace ringbuffer. Does it really need these ops?
> 

Besides my own ring buffer implementation in LTTng, at least Steven's
kernel/trace/ring_buffer.c (in mainline) use this too. We would need a
way to directly map to the same resulting behavior with per-cpu
variables.

In LTTng, I use local_cmpxchg, local_read, local_add and, in some
setups, local_add_return to manage the write counter and commit
counters. These per-cpu counters are kept in per-cpu buffer management
data allocated for each data collection "channel".

The current way I allocate this structure for all cpus is:

  chan->buf = alloc_percpu(struct ltt_chanbuf);

But note that each struct ltt_chanbuf contains a pointer to an array
containing each sub-buffer commit counters for the given buffer:

struct commit_counters {
        local_t cc;
        local_t cc_sb;                  /* Incremented _once_ at sb switch */
        local_t events;                 /* Event count */
};

struct ltt_chanbuf {
        struct ltt_chanbuf_alloc a;     /* Parent. First field. */
        /* First 32 bytes cache-hot cacheline */
        local_t offset;                 /* Current offset in the buffer */
        struct commit_counters *commit_count;
                                        /* Commit count per sub-buffer */
        atomic_long_t consumed;         /*
                                         * Current offset in the buffer
                                         * standard atomic access (shared)
                                         */
....

So I think accessing the "local_t offset" through percpu pointers should
be fine if I allocate struct ltt_chanbuf through the per cpu API.
However, I wonder how to deal with the commit_count counters, because
there is an indirection level.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
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