Re: [RFC patch 08/18] cnt32_to_63 should use smp_rmb()
- From: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxx>
- Date: Fri, 7 Nov 2008 12:17:03 -0500
* David Howells (dhowells@xxxxxxxxxx) wrote:
Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxx> wrote:
First off, using this macro with get_cycles() is simply buggy, because the
macro expects _perfect_ order of timestamps, no skew whatsoever, or
otherwise time could jump.
Erm... Why can't I pass it get_cycles()? Are you saying that sched_clock()
in MN10300 is wrong for it's use of get_cycles() with cnt32_to_63()?
Yes. Do you think the synchronization of the cycles counters is
_perfect_ across CPUs so that there is no possible way whatsoever that
two cycle counter values appear to go backward between CPUs ? (also
taking in account delays in __m_cnt_hi write-back...)
As I showed in my previous example, if you are unlucky enough to hit the
spot where the cycle counters go backward at the time warp edge, time
will jump of 2^32, so about 4.29s at 1GHz.
__x.lo = readl(cnt_lo); /* mmio read */
readl() might insert an extra barrier instruction. Not only that, io_addr
must be unsigned long.
If we expect the only correct use-case to be with readl(), I don't see
the problem with added synchronization.
Ah, right, then the parameters should be updated accordingly.
static inline u64 cnt32_to_63(unsigned long io_addr, u32 *__m_cnt_hi)
{
union cnt32_to_63 __x;
__x.hi = *__m_cnt_hi; /* memory read for high bits internal state */
rmb(); /*
* read high bits before low bits insures time
* does not go backward. Sync across
* CPUs and for interrupts.
*/
__x.lo = readl(io_addr); /* mmio read */
if (unlikely((s32)(__x.hi ^ __x.lo) < 0))
*__m_cnt_hi =
__x.hi = (__x.hi ^ 0x80000000) + (__x.hi >> 31);
return __x.val;
}
Mathieu
David
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Mathieu Desnoyers
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- References:
- Re: [RFC patch 08/18] cnt32_to_63 should use smp_rmb()
- From: Mathieu Desnoyers
- Re: [RFC patch 08/18] cnt32_to_63 should use smp_rmb()
- From: Andrew Morton
- [RFC patch 00/18] Trace Clock v2
- From: Mathieu Desnoyers
- [RFC patch 08/18] cnt32_to_63 should use smp_rmb()
- From: Mathieu Desnoyers
- Re: [RFC patch 08/18] cnt32_to_63 should use smp_rmb()
- From: Andrew Morton
- Re: [RFC patch 08/18] cnt32_to_63 should use smp_rmb()
- From: Nicolas Pitre
- Re: [RFC patch 08/18] cnt32_to_63 should use smp_rmb()
- From: David Howells
- Re: [RFC patch 08/18] cnt32_to_63 should use smp_rmb()
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- Re: [RFC patch 08/18] cnt32_to_63 should use smp_rmb()
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