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kern/subr_trap.c: repair the HPTS performance hack in userret()

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It wasn't functional as subr_trap.c doesn't include opt_inet.h.  Put a
better comment provided by gallatin@ in place of the old one.  The idea
is to use userret() as a cheap place to call a soft clock.  This approach
saves CPU on busy machines and saves power on idle machines.
An alternative would be to constantly schedule callouts.  Running with
neither callouts nor the soft clock ruins HPTS precision.

Reviewed by:		tuexen, rrs
Differential Revision:	https://reviews.freebsd.org/D42860

(cherry picked from commit e3cbc572f1541fdc18be9971d23e210d5018e662)
This commit is contained in:
Gleb Smirnoff 2023-12-04 10:19:46 -08:00
parent 2707135d09
commit 01df384133
4 changed files with 19 additions and 12 deletions
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20
sys/kern/subr_trap.c
View file

@ -76,6 +76,8 @@
#include <sys/epoch.h>
#endif
void (*tcp_hpts_softclock)(void);
/*
* Define the code needed before returning to user mode, for trap and
* syscall.
@ -127,16 +129,18 @@ userret(struct thread *td, struct trapframe *frame)
if (PMC_THREAD_HAS_SAMPLES(td))
PMC_CALL_HOOK(td, PMC_FN_THR_USERRET, NULL);
#endif
#ifdef TCPHPTS
/*
* @gallatin is adament that this needs to go here, I
* am not so sure. Running hpts is a lot like
* a lro_flush() that happens while a user process
* is running. But he may know best so I will go
* with his view of accounting. :-)
* Calling tcp_hpts_softclock() here allows us to avoid frequent,
* expensive callouts that trash the cache and lead to a much higher
* number of interrupts and context switches. Testing on busy web
* servers at Netflix has shown that this improves CPU use by 7% over
* relying only on callouts to drive HPTS, and also results in idle
* power savings on mostly idle servers.
* This was inspired by the paper "Soft Timers: Efficient Microsecond
* Software Timer Support for Network Processing"
* by Mohit Aron and Peter Druschel.
*/
tcp_run_hpts();
#endif
tcp_hpts_softclock();
/*
* Let the scheduler adjust our priority etc.
*/

1
sys/netinet/tcp_hpts.h
View file

@ -152,7 +152,6 @@ void __tcp_set_hpts(struct tcpcb *tp, int32_t line);
void tcp_set_inp_to_drop(struct inpcb *inp, uint16_t reason);
extern void (*tcp_hpts_softclock)(void);
void tcp_lro_hpts_init(void);
extern int32_t tcp_min_hptsi_time;

4
sys/netinet/tcp_lro.c
View file

@ -89,7 +89,6 @@ SYSCTL_NODE(_net_inet_tcp, OID_AUTO, lro, CTLFLAG_RW | CTLFLAG_MPSAFE, 0,
long tcplro_stacks_wanting_mbufq;
int (*tcp_lro_flush_tcphpts)(struct lro_ctrl *lc, struct lro_entry *le);
void (*tcp_hpts_softclock)(void);
counter_u64_t tcp_inp_lro_direct_queue;
counter_u64_t tcp_inp_lro_wokeup_queue;
@ -1262,8 +1261,7 @@ tcp_lro_flush_all(struct lro_ctrl *lc)
done:
/* flush active streams */
tcp_lro_rx_done(lc);
if (tcp_hpts_softclock != NULL)
tcp_hpts_softclock();
tcp_hpts_softclock();
lc->lro_mbuf_count = 0;
}

6
sys/sys/systm.h
View file

@ -380,6 +380,12 @@ void cpu_et_frequency(struct eventtimer *et, uint64_t newfreq);
extern int cpu_disable_c2_sleep;
extern int cpu_disable_c3_sleep;
extern void (*tcp_hpts_softclock)(void);
#define tcp_hpts_softclock() do { \
if (tcp_hpts_softclock != NULL) \
tcp_hpts_softclock(); \
} while (0)
char *kern_getenv(const char *name);
void freeenv(char *env);
int getenv_int(const char *name, int *data);