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tracing: Simplify the max length test when using the filtering temp buffer 

When filtering trace events, a temp buffer is used because the extra copy
from the temp buffer into the ring buffer is still faster than the direct
write into the ring buffer followed by a discard if the filter does not
match.

But the data that can be stored in the temp buffer is a PAGE_SIZE minus the
ring buffer event header. The calculation of that header size is complex,
but using the helper macro "struct_size()" can simplify it.

Link: https://lore.kernel.org/stable/CAHk-=whKbJkuVmzb0hD3N6q7veprUrSpiBHRxVY=AffWZPtxmg@mail.gmail.com/

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
This commit is contained in:
Steven Rostedt (VMware) 2021-06-09 18:04:58 -04:00
parent 99f4f5d623
commit faa76a6c28
1 changed files with 3 additions and 1 deletions
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4
kernel/trace/trace.c
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@ -2735,8 +2735,10 @@ trace_event_buffer_lock_reserve(struct trace_buffer **current_rb,
(trace_file->flags & (EVENT_FILE_FL_SOFT_DISABLED | EVENT_FILE_FL_FILTERED)) &&
(entry = this_cpu_read(trace_buffered_event))) {
/* Try to use the per cpu buffer first */
int max_len = PAGE_SIZE - struct_size(entry, array, 1);
val = this_cpu_inc_return(trace_buffered_event_cnt);
if ((len < (PAGE_SIZE - sizeof(*entry) - sizeof(entry->array[0]))) && val == 1) {
if (val == 1 && likely(len <= max_len)) {
trace_event_setup(entry, type, trace_ctx);
entry->array[0] = len;
return entry;