How can I see which CPU core a thread is running in?

The "top" command may help towards this, it does not have CPU-grouped list of threads but rather you can see the list of threads (probably for a single process) and which CPU cores the threads are running on by

top -H -p {PROC_ID}

then pressing f to go into field selection, j to enable the CPU core column, and Enter to display.

You can also use ps, something like this:

ps -mo pid,tid,%cpu,psr -p `pgrep BINARY-NAME`

The answer below is no longer accurate as of 2014

Tasks don't sleep in any particular core. And the scheduler won't know ahead of time which core it will run a thread on because that will depend on future usage of those cores.

To get the information you want, look in /proc/<pid>/task/<tid>/status. The third field will be an 'R' if the thread is running. The sixth from the last field will be the core the thread is currently running on, or the core it last ran on (or was migrated to) if it's not currently running.

31466 (bc) S 31348 31466 31348 34819 31466 4202496 2557 0 0 0 5006 16 0 0 20 0 1 0 10196934 121827328 1091 18446744073709551615 4194304 4271839 140737264235072 140737264232056 217976807456 0 0 0 137912326 18446744071581662243 0 0 17 3 0 0 0 0 0

Not currently running. Last ran on core 3.

31466 (bc) R 31348 31466 31348 34819 31466 4202496 2557 0 0 0 3818 12 0 0 20 0 1 0 10196934 121827328 1091 18446744073709551615 4194304 4271839 140737264235072 140737264231824 4235516 0 0 0 2 0 0 0 17 2 0 0 0 0 0

Currently running on core 2.

To see what the rest of the fields mean, have a look at the Linux kernel source -- specifically the do_task_stat function in fs/proc/array.c or Documentation/filesystems/stat.txt.

Note that all of this information may be obsolete by the time you get it. It was true at some point between when you made the open call on the file in proc and when that call returned.

The threads are not necessary to bound one particular Core (if you did not pin it). Therefore to see the continuous switching of the core you can use (a modified answer of Dmitry):

watch -tdn0.5 ps -mo pid,tid,%cpu,psr -p \`pgrep BINARY-NAME\`

For example:

watch -tdn0.5 ps -mo pid,tid,%cpu,psr -p \`pgrep firefox\`