On 05/29/20 11:08, Mel Gorman wrote: > On Thu, May 28, 2020 at 06:11:12PM +0200, Peter Zijlstra wrote: > > > FWIW, I think you're referring to Mel's notice in OSPM regarding the overhead. > > > Trying to see what goes on in there. > > > > Indeed, that one. The fact that regular distros cannot enable this > > feature due to performance overhead is unfortunate. It means there is a > > lot less potential for this stuff. > > During that talk, I was a vague about the cost, admitted I had not looked > too closely at mainline performance and had since deleted the data given > that the problem was first spotted in early April. If I heard someone > else making statements like I did at the talk, I would consider it a bit > vague, potentially FUD, possibly wrong and worth rechecking myself. In > terms of distributions "cannot enable this", we could but I was unwilling > to pay the cost for a feature no one has asked for yet. If they had, I > would endevour to put it behind static branches and disable it by default > (like what happened for PSI). I was contacted offlist about my comments > at OSPM and gathered new data to respond properly. For the record, here > is an editted version of my response; I had this impression too that's why I had a rather humble attempt. [...] > # Event 'cycles:ppp' > # > # Baseline Delta Abs Shared Object Symbol > # ........ ......... ........................ .............................................. > # > 9.59% -2.87% [kernel.vmlinux] [k] poll_idle > 0.19% +1.85% [kernel.vmlinux] [k] activate_task > +1.17% [kernel.vmlinux] [k] dequeue_task > +0.89% [kernel.vmlinux] [k] update_rq_clock.part.73 > 3.88% +0.73% [kernel.vmlinux] [k] try_to_wake_up > 3.17% +0.68% [kernel.vmlinux] [k] __schedule > 1.16% -0.60% [kernel.vmlinux] [k] __update_load_avg_cfs_rq > 2.20% -0.54% [kernel.vmlinux] [k] resched_curr > 2.08% -0.29% [kernel.vmlinux] [k] _raw_spin_lock_irqsave > 0.44% -0.29% [kernel.vmlinux] [k] cpus_share_cache > 1.13% +0.23% [kernel.vmlinux] [k] _raw_spin_lock_bh > > A lot of the uclamp functions appear to be inlined so it is not be > particularly obvious from a raw profile but it shows up in the annotated > profile in activate_task and dequeue_task for example. In the case of > dequeue_task, uclamp_rq_dec_id() is extremely expensive according to the > annotated profile. > > I'm afraid I did not dig into this deeply once I knew I could just disable > it even within the distribution. Could by any chance the vmlinux (with debug symbols hopefully) and perf.dat are still lying around to share? I could send you a link to drop them somewhere. Thanks -- Qais Yousef
