Software timers are complex and time consuming, and therefore should not be handled by the timer ISR. Rather they should be performed by a deferrable bottom-half softirq routine called TIMER_SOFTIRQ , and its routine is defined in kernel/time/timer.c:

static __latent_entropy void run_timer_softirq(struct softirq_action *h)
{
        struct timer_base *base = this_cpu_ptr(&timer_bases[BASE_STD]);

        base->must_forward_clk = false;

        __run_timers(base);
        if (IS_ENABLED(CONFIG_NO_HZ_COMMON) && base->nohz_active)
                __run_timers(this_cpu_ptr(&timer_bases[BASE_DEF]));
}