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3d9a0d2f82
While doing high throughput test on a BQL enabled NIC, I found a very high cost in ndo_start_xmit() when accessing BQL data. It turned out the problem was caused by compiler trying to be smart, but involving a bad MESI transaction : 0.05 │ mov 0xc0(%rax),%edi // LOAD dql->num_queued 0.48 │ mov %edx,0xc8(%rax) // STORE dql->last_obj_cnt = count 58.23 │ add %edx,%edi 0.58 │ cmp %edi,0xc4(%rax) 0.76 │ mov %edi,0xc0(%rax) // STORE dql->num_queued += count 0.72 │ js bd8 I got an incredible 10 % gain [1] by making sure cpu do not attempt to get the cache line in Shared mode, but directly requests for ownership. New code : mov %edx,0xc8(%rax) // STORE dql->last_obj_cnt = count add %edx,0xc0(%rax) // RMW dql->num_queued += count mov 0xc4(%rax),%ecx // LOAD dql->adj_limit mov 0xc0(%rax),%edx // LOAD dql->num_queued cmp %edx,%ecx The TX completion was running from another cpu, with high interrupts rate. Note that I am using barrier() as a soft hint, as mb() here could be too heavy cost. [1] This was a netperf TCP_STREAM with TSO disabled, but GSO enabled. Signed-off-by: Eric Dumazet <edumazet@google.com> Acked-by: Jesper Dangaard Brouer <brouer@redhat.com> Signed-off-by: David S. Miller <davem@davemloft.net>
106 lines
3.7 KiB
C
106 lines
3.7 KiB
C
/*
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* Dynamic queue limits (dql) - Definitions
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*
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* Copyright (c) 2011, Tom Herbert <therbert@google.com>
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*
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* This header file contains the definitions for dynamic queue limits (dql).
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* dql would be used in conjunction with a producer/consumer type queue
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* (possibly a HW queue). Such a queue would have these general properties:
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*
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* 1) Objects are queued up to some limit specified as number of objects.
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* 2) Periodically a completion process executes which retires consumed
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* objects.
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* 3) Starvation occurs when limit has been reached, all queued data has
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* actually been consumed, but completion processing has not yet run
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* so queuing new data is blocked.
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* 4) Minimizing the amount of queued data is desirable.
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*
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* The goal of dql is to calculate the limit as the minimum number of objects
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* needed to prevent starvation.
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*
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* The primary functions of dql are:
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* dql_queued - called when objects are enqueued to record number of objects
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* dql_avail - returns how many objects are available to be queued based
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* on the object limit and how many objects are already enqueued
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* dql_completed - called at completion time to indicate how many objects
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* were retired from the queue
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*
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* The dql implementation does not implement any locking for the dql data
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* structures, the higher layer should provide this. dql_queued should
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* be serialized to prevent concurrent execution of the function; this
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* is also true for dql_completed. However, dql_queued and dlq_completed can
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* be executed concurrently (i.e. they can be protected by different locks).
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*/
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#ifndef _LINUX_DQL_H
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#define _LINUX_DQL_H
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#ifdef __KERNEL__
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struct dql {
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/* Fields accessed in enqueue path (dql_queued) */
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unsigned int num_queued; /* Total ever queued */
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unsigned int adj_limit; /* limit + num_completed */
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unsigned int last_obj_cnt; /* Count at last queuing */
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/* Fields accessed only by completion path (dql_completed) */
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unsigned int limit ____cacheline_aligned_in_smp; /* Current limit */
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unsigned int num_completed; /* Total ever completed */
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unsigned int prev_ovlimit; /* Previous over limit */
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unsigned int prev_num_queued; /* Previous queue total */
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unsigned int prev_last_obj_cnt; /* Previous queuing cnt */
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unsigned int lowest_slack; /* Lowest slack found */
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unsigned long slack_start_time; /* Time slacks seen */
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/* Configuration */
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unsigned int max_limit; /* Max limit */
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unsigned int min_limit; /* Minimum limit */
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unsigned int slack_hold_time; /* Time to measure slack */
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};
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/* Set some static maximums */
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#define DQL_MAX_OBJECT (UINT_MAX / 16)
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#define DQL_MAX_LIMIT ((UINT_MAX / 2) - DQL_MAX_OBJECT)
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/*
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* Record number of objects queued. Assumes that caller has already checked
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* availability in the queue with dql_avail.
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*/
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static inline void dql_queued(struct dql *dql, unsigned int count)
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{
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BUG_ON(count > DQL_MAX_OBJECT);
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dql->last_obj_cnt = count;
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/* We want to force a write first, so that cpu do not attempt
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* to get cache line containing last_obj_cnt, num_queued, adj_limit
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* in Shared state, but directly does a Request For Ownership
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* It is only a hint, we use barrier() only.
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*/
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barrier();
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dql->num_queued += count;
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}
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/* Returns how many objects can be queued, < 0 indicates over limit. */
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static inline int dql_avail(const struct dql *dql)
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{
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return ACCESS_ONCE(dql->adj_limit) - ACCESS_ONCE(dql->num_queued);
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}
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/* Record number of completed objects and recalculate the limit. */
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void dql_completed(struct dql *dql, unsigned int count);
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/* Reset dql state */
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void dql_reset(struct dql *dql);
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/* Initialize dql state */
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int dql_init(struct dql *dql, unsigned hold_time);
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#endif /* _KERNEL_ */
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#endif /* _LINUX_DQL_H */
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