Hi Christian,
I agree with you. On the other hand, after you optimize the BO reservation lock, other locks still need optimization, right?
1. Locking itself is not cheap. 2. Waiting in lock is even more expensive. Thanks, Alex Bin Xie On 2017-06-23 09:01 AM, Christian König wrote:
The key point here is while optimizing this is nice the much bigger pile is the locking done for each BO.In other words even when we optimize all the other locks involved into atomics or RCU, the BO reservation lock will still dominate everything.One possible solution to this would be per process resources like I suggested multiple times now.Christian. Am 23.06.2017 um 13:37 schrieb Marek Olšák:I agree with you about the spinlock. You seem to be good at this. It's always good to do measurements to validate that a code change improves something, especially when the code size and code complexity has to be increased. A CPU profiler such as sysprof can show you improvements on the order of 1/10000th = 0.01% if you record enough samples. Sometimes you have to un-inline a function to make it visible there. If you see a function that takes 0.3% of CPU time and you optimize it down to 0.1% using the profiler as the measurement tool, you have evidence that the improvement is there and nobody can reject the idea anymore. It also proves that the code size increase is worth it. It's always "added code size and loss of simplicity" vs benefit. It's a transaction. You trade one for the other. You lose something to get something else. OK, we know the code complexity. Now, what's the benefit? Can you do some measurements? The accuracy of 1/10000th should be enough for anybody. I know the feeling when you spend many days working on something, adding 100s or 1000s of lines of code, solving many problems to get there and increasing code complexity significantly, and then you do the measurement and it doesn't improve anything. I know the feeling very well. It sucks. The frustration comes from the investment of time and getting no return on the investment. Many frustrations in life are like that. Marek On Fri, Jun 23, 2017 at 4:23 AM, axie <a...@amd.com> wrote:Hi Marek, So do you agree that spinlock disables CPU preemption, contrary to your original idea?If you have new reason that this patch does not improve, please speak out.Many patches in GPU driver aim at improving performance and powerefficiency. Does most patches submitted in AMDGPU requires a benchmarkingfirst?If all developers are required to always answer your questions when code review, I am afraid that most open source community developers cannot meetthat requirement and stop working on AMDGPU.To improve performance, there are many bottlenecks to clear. When the lastseveral bottlenecks are clear, the performance will show faster more significantly. My pass profiling experience told me that clearing a lock can improveperformance for some driver like 0.3% to much bigger percentage. It dependson many factors, even depends on the application itself.This is not the first bottleneck fixed. This is surely not the last one.Thanks, Alex Bin On 2017-06-22 07:54 PM, Marek Olšák wrote:That's all nice, but does it improve performance? Have you been able to measure some performance difference with that code? Were you targeting a specific inefficiency you had seen e.g. with a CPU profiler? Marek On Thu, Jun 22, 2017 at 8:19 PM, axie <a...@amd.com> wrote:To clarify, local IRQ is disabled by calling raw_local_irq_save(flags);Function __lock_acquire double checks that the local IRQ is really disabled. On 2017-06-22 01:34 PM, axie wrote:Hi Marek,Spin lock and spin unlock is fast. But it is not so fast compared withatomic, which is a single CPU instruction in x86.1. spinlock does NOT allow preemption at local CPU. Let us have a lookat how spin lock was implemented. static inline void __raw_spin_lock(raw_spinlock_t *lock) {preempt_disable(); <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<--This ismemory barrier operation too. spin_acquire(&lock->dep_map, 0, 0, _RET_IP_); LOCK_CONTENDED(lock, do_raw_spin_trylock, do_raw_spin_lock); }2. A function __lock_acquire called by spinlock. The function is solong that I would not attach all of it here. There is atomic operation inside and 12 meta data updates and 14 if statements and it calls quite some other functions. Note that it disable IRQ... static int __lock_acquire(struct lockdep_map *lock, unsigned int subclass, int trylock, int read, int check, int hardirqs_off, struct lockdep_map *nest_lock, unsigned long ip, int references, int pin_count) { struct task_struct *curr = current; struct lock_class *class = NULL; struct held_lock *hlock; unsigned int depth; int chain_head = 0; int class_idx; u64 chain_key; if (unlikely(!debug_locks)) return 0; /* * Lockdep should run with IRQs disabled, otherwise we could* get an interrupt which would want to take locks, which would* end up in lockdep and have you got a head-ache already? */if (DEBUG_LOCKS_WARN_ON(!irqs_disabled())) <<<<<<<<<<<<<<<DisableIRQ return 0; .... 3. Another function called by spinlock in a higher level: void lock_acquire(struct lockdep_map *lock, unsigned int subclass, int trylock, int read, int check, struct lockdep_map *nest_lock, unsigned long ip) { unsigned long flags; if (unlikely(current->lockdep_recursion)) return; raw_local_irq_save(flags); check_flags(flags); current->lockdep_recursion = 1;trace_lock_acquire(lock, subclass, trylock, read, check, nest_lock,ip); __lock_acquire(lock, subclass, trylock, read, check, irqs_disabled_flags(flags), nest_lock, ip, 0, 0); current->lockdep_recursion = 0; raw_local_irq_restore(flags); } Thanks, Alex Bin On 2017-06-22 12:27 PM, Marek Olšák wrote:On Thu, Jun 22, 2017 at 5:33 PM, Xie, AlexBin <alexbin....@amd.com> wrote:Hi Christian,In fact, the change from spinlock to atomic is quite painful. When I started, I thought it was easy but later I found there might be race condition here and there. Now I think the change looks more robust. In kernel source, there are several other drivers used the same trick.On the other hand, I think the logic itself might be optimized consideringthe locking. But I had spent quite some effort to maintain originallogic.It seems quite complicated and I don't know if there is anyperformance benefit. Spinlocks are nice because they allow preemption.It would be more interesting to merge the CS and BO_LIST ioctls intoone. Marek
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