libbsd select timeout issue

[RUI Zhengxin]

Hello everyone,




We meet an issue of select function timeout drift in rtems-libbsd5.1.

The select timeout can drift 1ms every 14000 seconds.




Kernel put the select thread to sleep queue in sleepq_set_timeout_sbt,

the expire value is calculated by this

expire = (sbt - SBT_1S + sbt_per_tick - 1) / sbt_per_tick;




sbt is the absolute time,

Precision is a constant value, no time drift.

but the sbt_per_tick is not an accurate value,

It can make the expire vaule bigger and bigger while sbt is increasing.




in rtems_bsd_initialize function,

rtems_bsd_sbt_per_watchdog_tick = SBT_1S / tps;

in our system the tick rate is set to 1000Hz,

The real value of rtems_bsd_sbt_per_watchdog_tick is 1<<32/1000=4294967.296

but in integer type the value is 4294967.

The error is 0.296/4294967 = 0.069ppm,

and it can increase the expire value 1ms every 14000 seconds.


Zhengxin
Best regards.
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