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Date: Thu, 3 Jan 2019 14:58:25 -0500 From: Pavel Tatashin <pasha.tatashin@...een.com> To: Marc Zyngier <marc.zyngier@....com> Cc: catalin.marinas@....com, Will Deacon <will.deacon@....com>, Andrew Morton <akpm@...ux-foundation.org>, rppt@...ux.vnet.ibm.com, Michal Hocko <mhocko@...e.com>, Ard Biesheuvel <ard.biesheuvel@...aro.org>, andrew.murray@....com, james.morse@....com, sboyd@...nel.org, linux-arm-kernel@...ts.infradead.org, LKML <linux-kernel@...r.kernel.org> Subject: Re: [PATCH v3 3/3] arm64: Early boot time stamps > I still think this approach is flawed. You provide the kernel with a > potentially broken sched_clock that may jump back and forth until the > workaround kicks in. Nobody expects this. > > Instead, I'd suggest you allow for a something other than local_clock() > to be used for the time stamping until a properly working sched_clock > gets registered. > > This way, you'll only impact the timestamps when running on a broken system. I think, given that on other platforms sched_clock() is already used early, it is not a good idea to invent a different clock just for time stamps. We could limit arm64 approach only for chips where cntvct_el0 is working: i.e. frequency is known, and the clock is stable, meaning cannot go backward. Perhaps we would start early clock a little later, but at least it will be available for the sane chips. The only question, where during boot time this is known. Another approach is to modify sched_clock() in kernel/time/sched_clock.c to never return backward value during boot. 1. Rename current implementation of sched_clock() to sched_clock_raw() 2. New sched_clock() would look like this: u64 sched_clock(void) { if (static_branch(early_unstable_clock)) return sched_clock_unstable(); else return sched_clock_raw(); } 3. sched_clock_unstable() would look like this: u64 sched_clock_unstable(void) { again: static u64 old_clock; u64 new_clock = sched_clock_raw(); static u64 old_clock_read = READ_ONCE(old_clock); /* It is ok if time does not progress, but don't allow to go backward */ if (new_clock < old_clock_read) return old_clock_read; /* update the old_clock value */ if (cmpxchg64(&old_clock, old_clock_read, new_clock) != old_clock_read) goto again; return new_clock; } Pasha
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