aarch64 and kqueue timer events

From: Matthew Grooms <mgrooms_at_shrew.net>
Date: Sat, 24 Jul 2021 20:18:57 -0500
Hi all,

I'm seeing some strange behavior with kqueue timers on my aarch64 host. 
Here is a simple test program that I've compiled on both amd64 and 
aarch64. It just sets up a simple kqueue timer with a loop that waits 
for the event to fire every 3 seconds ...

#include <err.h>
#include <time.h>
#include <stdio.h>
#include <unistd.h>
#include <sys/event.h>

int main( int argc, char *argv[] )
{
     int kq = kqueue();
     if( kq < 0 )
         err( 1, "failed to obtain kqueue handle" );

     struct kevent ke;
     EV_SET( &ke, 1, EVFILT_TIMER, EV_ADD | EV_ENABLE, 0, 3000, 0 );
     if( kevent( kq, &ke, 1, NULL, 0, NULL ) < 0 )
         err( 1, "failed to add kevent timer filter" );

     printf( "started at %i\n", (int)time(NULL) );

     while( true )
     {
         int r = kevent( kq, NULL, 0, &ke, 1, NULL );
         if( r < 0 )
             err( 1, "failed to read kevent list" );

         printf( "timer fired %i times at %i\n", r, (int)time(NULL) );
     }

     return 0;
}

When I compile and run the code on an amd64 host, everything works as 
expected. The event timer expires once every 3 seconds ...

mgrooms_at_amd64:~/devel$ cc test.cpp -o test && ./test
started at 1627175311
timer fired 1 times at 1627175314
timer fired 1 times at 1627175317
timer fired 1 times at 1627175320

However, when I compile and run the same code on an aarch64 host ( rpi4b 
), something very different happens. The event timer expires after 3 
seconds the first time. But each subsequent expiration is for exactly 
twice the defined interval, 6 seconds ...

mgrooms_at_aarch64:~/devel$ cc test.cpp -o test && ./test
started at 1625789207
timer fired 1 times at 1625789210
timer fired 1 times at 1625789216
timer fired 1 times at 1625789222

Any idea what's going on here? Seems like a bug in somewhere on the arm 
side.

Thanks in advance,

-Matthew
Received on Sun Jul 25 2021 - 01:18:57 UTC

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