On Fri, 2013-07-26 at 11:19 +0200, Johannes Berg wrote:
> My original though here was that we should be able to reserve (maximum)
> space on the per-CPU buffer, and then relinquish some of it after the
> tracepoint was written to the data, but I never had the time to check if
> that was possible to implement. It would make it a little inefficient at
> page boundaries, but that seems OK given that our maximum size is only
> ~100 bytes or so now.
>
Yes that would be trivial to implement. If the max buffer is ~100 bytes,
allocate 256 byte buffers per cpu. Also have a per cpu index, and then
have something like this:
index = local_add_return(this_cpu_ptr(trace_index), len);
if (index >= MAX_BUF) {
/* write tracepoint with failed buffer */
local_sub(this_cpu_ptr(trace_index));
return;
}
index -= len;
memcpy(this_cpu_ptr(trace_buf) + index, string, len);
/* write tracepoint with trace_buf */
local_sub(this_cpu_ptr(trace_index), len);
This way the trace_buf will work like a stack allocator. The
local_add_return() will reserve the space in the buffer such that if an
interrupt were to come in, it would allocate after that space. The check
for MAX_BUF tests for the case that the buffer was too big and had to
bail. Still trace that event to let the user (yourself) know you need a
bigger buffer.
-- Steve
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