softirqs: Combined CPU as part of the key is necessary to avoid amiss…#2804
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… value. In the environment of massive software interrupts. <idle>-0 [003] ..s1 106.421020: softirq_entry: vec=6 [action=TASKLET] <idle>-0 [000] ..s1 106.421063: softirq_entry: vec=3 [action=NET_RX] <idle>-0 [003] ..s1 106.421083: softirq_exit: vec=6 [action=TASKLET] Follow the above ftrace logs, we know the correct vec-6 start timestamp is replaced with incorrect vec-3. Because PID is idle-0. It will produce the wrong result after calculating delta.
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ismhong
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Dec 8, 2021
Currently, hardirqs use tid as key to store information while tracepoint irq_handler_entry. It works fine if irq is triggered while normal task running, but there is a chance causing overwrite issue while irq is triggered while idle task (a.k.a swapper/x, tid=0) running on multi-core system. Let's say there are two irq event trigger simultaneously on both CPU core, irq A @ core #0, irq B @ core #1, and system load is pretty light, so BPF program will get tid=0 since current task is swapper/x for both cpu core. In this case, the information of first irq event stored in map could be overwritten by incoming second irq event. Use tid and cpu_id together to make sure the key is unique for each event in this corner case. Please check more detail at merge request iovisor#2804, iovisor#3733.
yonghong-song
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Dec 9, 2021
Currently, hardirqs use tid as key to store information while tracepoint irq_handler_entry. It works fine if irq is triggered while normal task running, but there is a chance causing overwrite issue while irq is triggered while idle task (a.k.a swapper/x, tid=0) running on multi-core system. Let's say there are two irq event trigger simultaneously on both CPU core, irq A @ core #0, irq B @ core #1, and system load is pretty light, so BPF program will get tid=0 since current task is swapper/x for both cpu core. In this case, the information of first irq event stored in map could be overwritten by incoming second irq event. Use tid and cpu_id together to make sure the key is unique for each event in this corner case. Please check more detail at merge request #2804, #3733.
CrackerCat
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Jul 31, 2024
Currently, hardirqs use tid as key to store information while tracepoint irq_handler_entry. It works fine if irq is triggered while normal task running, but there is a chance causing overwrite issue while irq is triggered while idle task (a.k.a swapper/x, tid=0) running on multi-core system. Let's say there are two irq event trigger simultaneously on both CPU core, irq A @ core #0, irq B @ core iovisor#1, and system load is pretty light, so BPF program will get tid=0 since current task is swapper/x for both cpu core. In this case, the information of first irq event stored in map could be overwritten by incoming second irq event. Use tid and cpu_id together to make sure the key is unique for each event in this corner case. Please check more detail at merge request iovisor#2804, iovisor#3733.
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… value.
In the environment of massive software interrupts.
-0 [003] ..s1 106.421020: softirq_entry: vec=6 [action=TASKLET]
-0 [000] ..s1 106.421063: softirq_entry: vec=3 [action=NET_RX]
-0 [003] ..s1 106.421083: softirq_exit: vec=6 [action=TASKLET]
Follow the above ftrace logs, we know the correct vec-6 start timestamp is replaced with incorrect vec-3.
Because PID is idle-0. It will produce the wrong result after calculating delta.