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The extra cpu time granted would help in allowing the thread to >> complete executing the critical section and drop any locks without getting >> preempted. The thread would request this cpu time extension, by setting a >> bit in the restartable sequences(rseq) structure registered with the kernel. >> >> Kernel will grant a 50us extension on the cpu, when it sees the bit set. >> With the help of a timer, kernel force preempts the thread if it is still >> running on the cpu when the 50us timer expires. The thread should yield >> the cpu by making a system call after completing the critical section. >> >> Suggested-by: Peter Ziljstra >> Signed-off-by: Prakash Sangappa >> --- > >> diff --git a/include/uapi/linux/rseq.h b/include/uapi/linux/rseq.h >> index c233aae5eac9..900cb75f6a88 100644 >> --- a/include/uapi/linux/rseq.h >> +++ b/include/uapi/linux/rseq.h >> @@ -26,6 +26,7 @@ enum rseq_cs_flags_bit { >> RSEQ_CS_FLAG_NO_RESTART_ON_PREEMPT_BIT = 0, >> RSEQ_CS_FLAG_NO_RESTART_ON_SIGNAL_BIT = 1, >> RSEQ_CS_FLAG_NO_RESTART_ON_MIGRATE_BIT = 2, >> + RSEQ_CS_FLAG_DELAY_RESCHED_BIT = 3, >> }; >> >> enum rseq_cs_flags { >> @@ -35,6 +36,8 @@ enum rseq_cs_flags { >> (1U << RSEQ_CS_FLAG_NO_RESTART_ON_SIGNAL_BIT), >> RSEQ_CS_FLAG_NO_RESTART_ON_MIGRATE = >> (1U << RSEQ_CS_FLAG_NO_RESTART_ON_MIGRATE_BIT), >> + RSEQ_CS_FLAG_DELAY_RESCHED = >> + (1U << RSEQ_CS_FLAG_DELAY_RESCHED_BIT), >> }; >> >> /* >> @@ -128,6 +131,10 @@ struct rseq { >> * - RSEQ_CS_FLAG_NO_RESTART_ON_MIGRATE >> * Inhibit instruction sequence block restart on migration for >> * this thread. >> + * - RSEQ_CS_DELAY_RESCHED >> + * Request by user task to try delaying preemption. With >> + * use of a timer, extra cpu time upto 50us is granted for this >> + * thread before being rescheduled. >> */ >> __u32 flags; > > So while it works for testing, this really is a rather crap interface > for real because userspace cannot up front tell if its going to work or > not. I'm also concerned about this. Using so far unused bits within those flags is all nice in terms of compactness, but it does not allow userspace to query availability of the feature. It will set the flag and trigger warnings on older kernels. There are a few approaches we can take to make this discoverable: A) Expose the supported flags through getauxval(3), e.g. a new AT_RSEQ_CS_FLAGS which would contain a bitmask of the supported rseq_cs flags, or B) Add a new "RSEQ_QUERY_CS_FLAGS" flag to sys_rseq @flags parameter. It would return the supported rseq_cs flags. Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. https://www.efficios.com