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cpu在内核中运行时并不是处处不可抢占的，内核中存在一些空隙，在这时进行抢占是安全的，内核抢占补丁的基本原理就是将smp可并行的代码段看成是可以进行内核抢占的区域。
lm6 ,/ [ 此文转贴于 贵 州 学 习 网 电脑课堂LINUX教程 http://WWW.gzu521.COM ] lm6 ,/ 2.4内核正好细化了多cpu下的内核线程同步机构，对不可并行的指令块用spinlock和rwlock作了细致的表示，该补丁的实现可谓水到渠成。具体的方法就是在进程的任务结构上增加一个preempt_count变量作为内核抢占锁，它随着spinlock和rwlock一起加锁和解锁。当preempt_count为0时表示可以进行内核调度。内核调度器的入口为preempt_schedule()，它将当前进程标记为task_preempted状态再调用schedule()，在task_preempted状态，schedule()不会将进程从运行队列中删除。 下面是内核抢占补丁的主要代码示意： arch/i386/kernel/entry.s: preempt_count = 4 # 将task_struct中的flags用作preempt_count,flags被移到了别 的位置 ret_from_exception: # 从异常返回 #ifdef config_smp get_current(%ebx) movl processor(%ebx),%eax shll $config_x86_l1_cache_shift,%eax movl symbol_name(irq_stat)(,%eax),%ecx # softirq_active testl symbol_name(irq_stat)+4(,%eax),%ecx # softirq_mask #else movl symbol_name(irq_stat),%ecx # softirq_active testl symbol_name(irq_stat)+4,%ecx # softirq_mask #endif jne handle_softirq #ifdef config_preempt cli incl preempt_count(%ebx) # 异常的入口没有禁止内核调度的指令,与ret_from_intr 匹配一下 #endif entry(ret_from_intr) # 硬件中断的返回 get_current(%ebx) #ifdef config_preempt cli decl preempt_count(%ebx) # 恢复内核抢占标志 #endif movl eflags(%esp),%eax # mix eflags and cs movb cs(%esp),%al testl $(vm_mask | 3),%eax # return to vm86 mode or non-supervisor? jne ret_with_reschedule #ifdef config_preempt cmpl $0,preempt_count(%ebx) jnz restore_all # 如果preempt_count非零则表示禁止内核抢占 cmpl $0,need_resched(%ebx) jz restore_all # movl symbol_name(irq_stat)+irq_stat_local_bh_count cpu_indx,%ecx addl symbol_name(irq_stat)+irq_stat_local_irq_count cpu_indx,%ecx jnz restore_all incl preempt_count(%ebx) sti call symbol_name(preempt_schedule) jmp ret_from_intr # 新进程返回,返回ret_from_intr恢复抢占标志后再返回 #else jmp restore_all #endif align handle_softirq: #ifdef config_preempt cli get_current(%ebx) incl preempt_count(%ebx) sti #endif call symbol_name(do_softirq) jmp ret_from_intr align reschedule: call symbol_name(schedule) # test jmp ret_from_sys_call include/asm/hw_irq.h: ... #ifdef config_preempt #define bump_contex_switch_lock \ get_current \ "incl 4(%ebx)\n\t" #else #define bump_contex_switch_lock #endif #define save_all \ 硬件中断保护入口现场 "cld\n\t" \ "pushl %es\n\t" \ "pushl %ds\n\t" \ "pushl %eax\n\t" \ "pushl %ebp\n\t" \ "pushl %edi\n\t" \ "pushl %esi\n\t" \ "pushl %edx\n\t" \ "pushl %ecx\n\t" \ "pushl %ebx\n\t" \ "movl $" str(__kernel_ds) ",%edx\n\t" \ "movl %edx,%ds\n\t" \ "movl %edx,%es\n\t" \ bump_contex_switch_lock # 硬件中断的入口禁止内核抢占 include/linux/spinlock.h: #ifdef config_preempt #define switch_lock_count() current->preempt_count #define in_ctx_sw_off() (switch_lock_count().counter) 判断当前进程的抢占计数 是否非零 #define atomic_ptr_in_ctx_sw_off() (&switch_lock_count()) #define ctx_sw_off() \ 禁止内核抢占 do { \ atomic_inc(atomic_ptr_in_ctx_sw_off()); \ 当前进程的内核抢占计数增1 } while (0) #define ctx_sw_on_no_preempt() \ 允许内核抢占 do { \ atomic_dec(atomic_ptr_in_ctx_sw_off()); \ 当前进程的内核抢占计数减1 } while (0) #define ctx_sw_on() \ 允许并完成内核抢占 do { \ if (atomic_dec_and_test(atomic_ptr_in_ctx_sw_off()) && \ current->need_resched) \ preempt_schedule(); \ } while (0) #define spin_lock(lock) \ do { \ ctx_sw_off(); \ 进入自旋锁时禁止抢占 _raw_spin_lock(lock); \ } while(0) #define spin_trylock(lock) ({ctx_sw_off(); _raw_spin_trylock(lock) ? \锁定并 测试原来是否上锁 1 : ({ctx_sw_on(); 0;});}) #define spin_unlock(lock) \ do { \ _raw_spin_unlock(lock); \ ctx_sw_on(); \ 离开自旋锁时允许并完成内核抢占 } while (0) #define read_lock(lock) ({ctx_sw_off(); _raw_read_lock(lock);}) #define read_unlock(lock) ({_raw_read_unlock(lock); ctx_sw_on();}) #define write_lock(lock) ({ctx_sw_off(); _raw_write_lock(lock);}) #define write_unlock(lock) ({_raw_write_unlock(lock); ctx_sw_on();}) #define write_trylock(lock) ({ctx_sw_off(); _raw_write_trylock(lock) ? \ 1 : ({ctx_sw_on(); 0;});}) ... |
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