Hi Thomas, On Thu, Jun 14, 2018 at 11:08:04AM +0200, Thomas Gleixner wrote: > On Thu, 14 Jun 2018, Feng Tang wrote: > > On Wed, May 30, 2018 at 05:20:58PM +0800, Feng Tang wrote: > > > Currently printk timestamp mostly come from the sched_clock which > > > depends on the clock setup, so there are many kernel logs started > > > with "[ 0.000000] " before the clock is calibrated. > > > > > > This patch will provide an debug option for specific platform to > > > provide a early boot time clock, so that we can have time info in > > > kernel log much earlier, which can show the time info for the early > > > kernel boot, and make boottime tuning/optimization easier (boot time > > > is critical for phone/tablet and embedded devices). > > > > > > Capable platform only need to setup the "boot_printk_clock_fn" > > > which could return time in nano seconds. > > > > > > Together with a TSC patch on x86 system, we have easily captured > > > some early boottime killer like unwind_init() which takes about > > > 300ms in boot phase. > > > > Hi Petr and all, > > > > As the 2/2 tsc related patch is still under review/discussion, can > > we consider taking this first? As this may benefit other archs. > > For example, Intel Curie platform has an always-on 32KHz osc clock, > > which is accurate but low frequency, and it could be used as > > early printk timestamp until the high-resolution timer is initialized > > and used as sched_clock. Don't know if ARM or other platforms > > have similar use case. > > Can we please _NOT_ add half sorted core stuff in a hurry?
Got it. The status for the 2/2 tsc patch is Peter suggested to bring the tsc_init() earlier, with that this 1/2 is not needed for x86. I followed Peter's suggestion and worked out a hack version with which the tsc inits fine and printk timestamp works much earlier (detail is in the 2/2 thread). But there is one blocker, the native_sched_clock()(tsc.c) use one staic key __use_tsc and sched_clock_cpu()(clock.c) use another "__sched_clock_stable", which cannot be used so early, one problem is the static_branch_enable() will use pageing related code while the paging is not setup ready yet. I haven't got idea to solve this first problem regarding static key. Thanks, Feng
