Messages in this thread | | | Date | Tue, 19 May 2009 17:03:46 -0400 | From | Jason Baron <> | Subject | [PATCH 0/3] tracepoints: delay argument evaluation |
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hi,
After disassembling some of the tracepoints, I've noticed that arguments that are passed as macros or that perform dereferences, evaluate prior to the tracepoint on/off check. This means that we are needlessly impacting the off case.
I am proposing to fix this by adding a macro that first checks for on/off and then calls 'trace_##name', preserving type checking. Thus, callsites have to move from:
trace_block_bio_complete(md->queue, bio);
to:
tracepoint_call(block_bio_complete, md->queue, bio);
I've tried '__always_inline', but that did not fix this issue. Obviously this change will require changes to all the callsites. But, that shouldn't be very hard, I've already included the scheduler and block changes with this patch. I think its important to minimize code execution in the off case, and thus going through all the callsites is well worth it. If we agree on this change, I can change the rest in very short order.
Below I'm also showing the assembly in the 'dec_pending()' function before and after this change to show the difference it makes. The arguments to the tracepoint are as above, 'md->queue' and 'bio'. Notice the 2 extra instructions, before the initial 'je', that could be moved after the 'je'.
before:
ffffffff8137b2a3: 83 3d de 90 4b 00 00 cmpl $0x0,0x4b90de(%rip) # ffffffff81834388 <__tracepoint_block_bio_complete+0x8> ffffffff8137b2aa: 49 8b 45 50 mov 0x50(%r13),%rax ffffffff8137b2ae: 48 89 45 d0 mov %rax,-0x30(%rbp) ffffffff8137b2b2: 74 1f je ffffffff8137b2d3 <dec_pending+0x101> ffffffff8137b2b4: 48 8b 1d d5 90 4b 00 mov 0x4b90d5(%rip),%rbx # ffffffff81834390 <__tracepoint_block_bio_complete+0x10> ffffffff8137b2bb: 48 85 db test %rbx,%rbx ffffffff8137b2be: 74 13 je ffffffff8137b2d3 <dec_pending+0x101> ffffffff8137b2c0: 4c 89 f6 mov %r14,%rsi ffffffff8137b2c3: 48 8b 7d d0 mov -0x30(%rbp),%rdi ffffffff8137b2c7: ff 13 callq *(%rbx) ffffffff8137b2c9: 48 83 c3 08 add $0x8,%rbx ffffffff8137b2cd: 48 83 3b 00 cmpq $0x0,(%rbx) ffffffff8137b2d1: eb eb jmp ffffffff8137b2be <dec_pending+0xec> ffffffff8137b2d3: 44 89 fe mov %r15d,%esi
after:
ffffffff8137b2a3: 83 3d de 90 4b 00 00 cmpl $0x0,0x4b90de(%rip) # ffffffff81834388 <__tracepoint_block_bio_complete+0x8> ffffffff8137b2aa: 74 27 je ffffffff8137b2d3 <dec_pending+0x101> ffffffff8137b2ac: 49 8b 45 50 mov 0x50(%r13),%rax ffffffff8137b2b0: 48 8b 1d d9 90 4b 00 mov 0x4b90d9(%rip),%rbx # ffffffff81834390 <__tracepoint_block_bio_complete+0x10> ffffffff8137b2b7: 48 89 45 d0 mov %rax,-0x30(%rbp) ffffffff8137b2bb: 48 85 db test %rbx,%rbx ffffffff8137b2be: 74 13 je ffffffff8137b2d3 <dec_pending+0x101> ffffffff8137b2c0: 4c 89 f6 mov %r14,%rsi ffffffff8137b2c3: 48 8b 7d d0 mov -0x30(%rbp),%rdi ffffffff8137b2c7: ff 13 callq *(%rbx) ffffffff8137b2c9: 48 83 c3 08 add $0x8,%rbx ffffffff8137b2cd: 48 83 3b 00 cmpq $0x0,(%rbx) ffffffff8137b2d1: eb eb jmp ffffffff8137b2be <dec_pending+0xec> ffffffff8137b2d3: 44 89 fe mov %r15d,%esi
thanks,
-Jason
Jason Baron (3): -add wrapper so we don't have argument resolution overhead -add scheduler wrapper calls -add block layer trace wrappers
block/blk-core.c | 27 ++++++++++++++------------- block/elevator.c | 6 +++--- drivers/md/dm.c | 7 ++++--- fs/bio.c | 2 +- include/linux/tracepoint.h | 20 +++++++++++++++++++- kernel/exit.c | 6 +++--- kernel/fork.c | 2 +- kernel/kthread.c | 4 ++-- kernel/sched.c | 10 +++++----- kernel/signal.c | 2 +- mm/bounce.c | 2 +- 11 files changed, 54 insertions(+), 34 deletions(-)
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